There are ways to manipulate this in his favor, such as stating the opposite of what he wants in the simplest way possible, but because as little as possible is changed, this can be tricky in some cases since only one detail of his statement must be false in order to render the entire statement technically false.

Bob wants actually to state things not as simply as possible, since as you mentioned, this leads to several possible outcomes. He wants to state things as precise as possible.

"My net worth is at most 10 dollars", has the logical (and only possible) opposite "My net worth is more than 10 dollars" (and Bob will have net worth of 10.01 dollars if taking the path least resistance).

"The set of people who distrusts me is non empty" has opposite "The set of people who distrusts me is empty", i.e. no one distrusts Bob...

You misunderstand trust: I trust Bob completely.

A lack of trust comes from unpredictability and betrayal. Bob hasn't betrayed me because I trust him completely: he will always speak falsely. Frankly, that makes him a great deal more honest than many people I know, people whose honesty is unpredictable and therefore untrustworthy.

Me? I'm dishonest, and a dishonest man you can always trust to be dishonest. Honestly. It's the honest ones you want to watch out for, because you can never predict when they're going to do something incredibly... stupid. — Jack Sparrow

There are seconds during this week/month/year that you will not trust me a bit. --Bob

Easy. Bob can gain people's trust through his actions. If he acts to help people, that will show he's trustworthy.

Don't Talk

"I have, at most, level 4 ASLPI proficiency level."

Then Bob can just play deaf. And if necessary, make himself temporarily deaf. Communicate with sign language, or writing if necessary. But it may not be necessary, since he can just tell someone that they are not sign proficient either.

Bob, the pessimistic super doctor

Work in a hospital. Always complain. "This patient won't survive." "That will leave scar." "It will take at least five hours of physical therapy before she can walk again." "You have cancer."

So no-one will trust what Dr. Bob says. But they will trust that he can save any patient, cure any illness.

Beyond that, go into the labs and complain that their research isn't ready. "YOu haven't finished your cure for the cold." "It only works on specific strains of the virus." "It has side effects." "It takes multiple doses." "It takes more than a day to work." "It costs more than 5 dollars to manufacture a dose." "It doesn't taste like chocolate."

Pretend he's mute, keep his mouth shut and write everything down.

My answer assumes that Bob can not predict HOW things will change, just that they will change, in subtle ways. Also, I am assuming that Bob can and will voluntarily answer your questions. I am also assuming that this works for any PREDICTIONS Bob makes. That is, any prediction that he makes will never be true. I am also assuming that Bob, in god faith, can tell the truth as it was at the time of him saying it.

I am not sure if the issue is in not being able to trust that the answer Bob gives you is the truth, but in being able to absolutely trust that the answer Bob gives you is NOT the truth.

If a betting man knew that Bob's answer could always be trusted to ultimately NOT be true, then a betting man could make a lot of money.

'Bob, what team will win the Series?' Then bet AGAINST the team he says. 'I will bet you that your favorite team x will NOT win the Series'. 'Oh, come ON, they are sure to win the series. You're ON, man, you're ON'.

Of course, a person with criminal intent would WANT him to tell the truth, so that it would become the UNtruth.

'Bob, try that door and tell me if it is locked?' In which case, you WANT him to tell the truth, that it is locked, so that it then becomes the UNtruth.

'Bob, is that watchman who is looking at us paying attention to us?'

'Bob, is that merchant charging for his goods?'

'Bob, will you tell the judge the truth about my guilt?' It doesn't matter if the judge believes him or not, but as soon as Bob says I am guilty, I am no longer guilty.

It would seem to me, that the utility and the advantage of having Bob as a friend would not be in what Bob says is the truth, but what HAPPENS in response to his telling the truth. That it reliably and immediately becomes the UNtruth in some way.

In day to day conversations as a friend, one could easily accommodate his eccentricities, if one knew absolutely that whatever he said, even if said in good faith, would be wrong.

'Bob, how much money do you have?' Bob: 'I have ten dollars'. So, if Bob answered truthfully in good faith, you know he has approximately ten dollars, but not exactly ten dollars.

'Bob, what time is it?' Bob: 'It is ten o'clock'. So again, if Bob is answering truthfully in good faith, and tells you the correct time, then you know it is AROUND ten o'clock but not exactly ten o'clock.

If he always answers with an approximate answer, that is close to the truth, you of course can always accommodate. The trick is to learn to ask Bob the question in the right way.

So those who perhaps find utility in Bob, and want to capitalize on his powers, but also want to have a good relationship with him on good faith, would have no problems as long as they always knew his good-faith answers were APPROXIMATE answers, and his devious faith answers were always the truth as he knew it at the time of his response. You and Bob would know the answer would be immediately incorrect as soon as he gave the response.

And, of course, there is the situations along the lines of: Bob: 'I need to go to the bathroom right now' means that, if it were the truth, Bob NO LONGER has to go to the bathroom right now, but he WILL have to go to the bathroom AGAIN (is that the right term) shortly.

in such a way, as long as the other person wanted to have a relationship with Bob, and Bob always responded in good faith with the truth, but both of you knew it was no longer the truth but approximately the truth, the relationship would work out.

So, really, it is about how much the OTHER person wants to have a good relationship with Bob, and is willing to accommodate, provided Bob enters the relationship in good faith.

When Bob wants to communicate, he doesn't make statements. He asks questions that lead people to the correct conclusions or uses imperatives.

Mr. X: Bob, what is behind this door?

Bob: Would you believe there's a
tiger behind the door? Don't open the door.

Mr. X: Bob, where's the report you were supposed to hand in?

Bob: Where would you expect it to be?

Mr. X: On my desk in my in-box.

Bob: It's not there. Or is it?

Mr. X: Bob, why do you always ask questions instead of directly
telling what you want?

Bob: Hypothetically, if an honest person was
incapable of making a true statement because of a cursed super-power,
how do you think such a person would communicate?

You kind of answered your own question, he can't remove this superpower by stating he has it so he can go up to anyone, explain the way the power works and then demonstrate it using some easy cases. People will be skeptical at first but will believe him after enough convincing examples.

It is really simple.

Don't try to make them believe you. Always tell the lie that they won't believe to get them to realize the truth.

If your friend asks if you want to head out to dinner and you do, actually want to, say "no." Your friend will know that you can't tell the truth and know what you are saying.

Once enough people know about the power, everyone will know that he has to speak that way.

Also, he would be in high demand for anything that requires safety.

Any time he gets on an airplane, all he has to say is "this plane will not land safely."

He can be the benevolent doomsayer.

How linguistically savvy is this superpower? How linguistically savvy is Bob?

I'm assuming here that Bob wants to communicate the truth and wants to be trusted. I'm also going to be treating the superpower as an antagonist to this goal.

If the superpower can recognize the intent of his statements and will change the truth of the part he intends to communicate, he's kind of stuck (although maria_c's answer is a good one for getting as close as possible to the truth).

If, however, the superpower only analyzes his statements in terms of syntax and semantics, Bob can work around the limitation in various ways.

First, he can use questions instead of statements. While declarative sentences have a truth value (as long as they aren't paradoxes), interrogative sentences do not. So if he wanted to tell you that Madrid is the capital of Spain, he could ask you "Did you know that Madrid is the capital of Spain?"

Second, he could always tell you what he wants you to know in a content clause. This will still involve a declarative sentence, but by selecting the main clause carefully he can make a sentence that is already false (and therefore exempt from the power), but that nevertheless has a true content clause. For example: "No one knows that Madrid is the capital of Spain." People already know that Madrid is the capital of Spain, therefore the statement is false. It can't be further falsified and the superpower won't touch it.

NOTE: This answer assumes Bob cannot make people trust him simply by saying "You don't trust me".

It's very easy to get people to trust him. Simply announce in a loud voice the opposite of what you want and soon people will pick up that the opposite will happen. If you want people to notice faster do it in a casino.

It will take some time to fine-tune what to say, but this is essentially a wish granting power. You just have to phrase it a little weird.

Your friends will all want you to say things like

My company's value will go down

Or

It'll land on black

Or

Your marriage won't last

Several economic indicators are like this. Things like more stocks being issued (IPOs) and more credit being available happens before a stock market crash. People watch these and try to time the market (but rarely succeed).

The real curse is to say the truth but have no one believe you like Cassandra

In @KaspervandenBerg 's answer:

There are seconds during this week/month/year that you will not trust me a bit. --Bob

due to least resistance, you end up with people only trusting bob slightly more than a bit.

I propose this:

You trust me less than you trust everyone else.

In a computer game "Divinity: Original Sin II" there was one character, a talking mouse, who was cursed and as a result could only tell lies. When you meet him, he wants you to remove the curse, but, of course, he can't simply tell you about it, because that would be the truth he cannot tell. So he simply reverses everything and says something like "I am not a mouse. I don't have this curse that makes me always tell a lie. I don't think you could help me and remove the curse by doing so and so... Doing so and so. It wouldn't help. Got it?" Can't your Bob just do the same?

Bob can be a superhero and become rich really easy.

All he needs to do is to spread conspiracy theories.

Bob says the Earth is flat. The Earth assumes a spherical geometry. Had Bob not used his power, we would eventually be suffocated by the giant elephants' magical freezing farts which keep the frozen barrier st the rim in place, or we would eventually be smashed by the cosmic turtle's sexual partner.

Bob says aliens are abducting people and probing their cavities. Now the aliens can't do that - Bob is a one man X-com!

Bob says planes leave out chemtrails that are used for mind control. Now they don't anymore! Take that, Illuminatti!

And so on. The only conspiracy theories he can't defeat are vaccine and GMO related ones, because those can actually cause direct harm to people.

So most everyone from the average Joe to the eggheads of our time will not trust Bob, but that's not a problem. He will have the undisputed and unwavering trust and loyalty of basically every Infowars follower. He can even take over Alex Jones's place and make huge loads of cash that way.

Heck, Bob could even easily become the next republican president, and he would save the world from climate change by claiming that coal is cleaner than solar and wind power.

Simple, there's a class of people that do similar all the time: Be a denialist speaker.

Many people will believe what's most convenient if it's said with authority and minimal requirement for thinking or action on their part. Just look at flat earthers, climate deniers, anti-vaxxers, and similar.

Just support wrong things with momentum, and everything suppporting that wrong thing will believe him.

Couldn't Bob simply tell someone he meets like some girl he likes very much and after many different machinations and presumed falsehoods he'd say something like "You simply do not understand me" by accident? In that way someone actually WOULD understand Bob's superpower without being harmed? Then communication could be carried out albeit in a very curiously and perhaps very humorous way. Just a thought.

It would be interesting to have a scene of an argument between Joe and Bob perhaps... I don't know how you'd do it but it could amount to a superpower Abbot and Costello routine.

He lies all the time. Even if says the truth, it changes to an untruth because he said it. So, the truth is something that is not a fact when he says it is. What he says is either a lie or about to be a lie. But he wants to build people’s trust. He begins by speaking only about the trifle things. It will be noticed that Bob’s lies do not touch anything that should be unchanged because the benefits of everyone who Bob wants to trust him depend on it. He learns to know what benefits people and stays away from mentioning those things. He must be very careful, for any of his statement can catch the fringes of the important and shift it. Other than that, he can speak lies as much as he wants. About the things that they have no idea of, for example. He earns people’s trust into a fact that he would never speak about the important things. What Bod is talking about is not important. They can take it to the bank. Once they think so, he can return and attack benefits. There is nothing important now, for he seems to speaks about everything. Either an unimportant thing is true or false does not make difference under a certain point. The only thing they know for sure that Bob would never speak about it if it would be of any importance. The only thing that is left true is Bob.

Bob says a variation of

"[Person's name] doesn't know about my inability to tell the truth"

Then his power will kick in and cause that person to know about Bob's power somehow. Bob can work out the exact phrasing with trial and error.