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My friend and I, neither native English speaker, had an argument about the meaning/implication over this sentence:

"I’d sit this one out, Cap," Natasha said.

My friend believed that it means :

"Natasha told Cap that she would not join the fight between Thor and Iron Man, It's his (Cap) turn to do so."

I believe that this is a suggestion as :

"I would sit this one out if I were you, cap", Natasha said.

I feel like it's an omission of "if I were you" in spoken language, BUT I have no idea how this idea comes from.

So which of us is wrong? Or both wrong? Could anyone help to explain the " 'd " in this sentence ?

Here is the sentence in the context:

With Iron Man and Captain America out of commission, the blond warrior
grabbed Loki around the neck, and before anyone could do anything
about it, he raised the hammer and jumped back out of the Quinjet,
disappearing into the storm. “Another Asgardian?” Natasha called from
the cockpit.

“That guy’s a friendly?” Steve asked. It was hard to believe.

“Doesn’t matter,” Iron Man said. “If he frees Loki—or kills him—the
Tesseract’s lost.”

“Stark, we need a plan of attack!” Steve said as Iron Man stomped
toward the open gangway.

“I have a plan,” Iron Man said over his shoulder. “Attack.”

Then he rocketed out of the ship.

Steve was amazed at the speed at which Tony moved. He grabbed a
parachute and strapped it on.

Natasha looked at him skeptically. They were thousands of feet above
land, the Quinjet was moving at a supersonic clip, and—as far as she
knew—Captain America couldn’t fly.

“I’d sit this one out, Cap,” she said.

“I don’t see how I can,” Steve said.

“These guys come from legend. They’re basically gods.”

Maybe Steve was old-fashioned, but he didn’t think so. “There’s only
one God, ma’am. And I’m pretty sure he doesn’t dress like that.”

The Avengers I

Further information: This is the subjunctive mood. It expresses things that aren't real: possibilities, hypotheticals, wishes, that sort of thing. (The "normal" mood, for things that are real, is called the "indicative".)

"Would" is a modal verb, used instead of "will" to mark the subjunctive. In this example, Natasha can use the indicative to talk about herself:

I'll [I will] sit this one out, Cap.

But she's not saying that. She gives Steve her opinion on what Steve should do. As the asker rightly guessed, she is implying that if she were him, she would not go.

Obviously, Natasha is not Steve. That's why she uses the subjunctive: she is talking about a hypothetical situation in which Natasha is taking Steve's place and making his choices.

I'd [I would] sit this one out, Cap.

The subjunctive often goes together with a condition: "if [condition], then I would...". The reader is expected to do exactly what you did, and guess the unspoken condition that Natasha is implying. We can do this because "if I were you" is a very common idiom used to express opinions like this.

By the way, the English subjunctive is a pain even for native speakers. Unless they've been specifically taught about it, native speakers often don't even know it exists. This is partly because it doesn't have any special form to itself. As well as subjunctive uses, "would" can be a past tense ("when I was young, we would go down to the arcade after school"). And other verbs in the subjunctive mood look exactly the same as their present plural, or past, forms—"were" is an example used above.

I am you. (Indicative)

...if I were you. (Subjunctive)

We were in the Quinjet. (Indicative? Subjunctive? You have to guess from context.)

"I'd" means "I would".

So that would surely mean "I would sit this one out if I were you, Cap."

If she needed to simply say that she's not getting into the fight, she could've just used "I will" instead of "I would". Besides, Steve replied to her with "I don’t see how I can." ( meaning he can't), clearly pointing out that she was suggesting him to stay out.

"Natasha told Cap that she would not join the fight between Thor and Iron Man, it's his (Cap) turn to do so."

This is definitely NOT the message of the fragment presented.

“I’d sit this one out, Cap,” she said.

with the same meaning as:

"I would sit this one out if I were you, cap", Natasha said.

(your own guess - which is correct)

Judging the sentence, the conclusion is:

• Natasha has no plan to joint the action;


• she advises Cap to also not join the action.

I am not entirely sure what is the action:

• the fight coming (“I have a plan,” Iron Man said over his shoulder. “Attack.”);


• jumping out of the Quinjet (Captain America couldn’t fly).

Regarding I'd, @BellaSwan made a a good point: I'd = I would.

For the difference between will and would, you may read: Correct usage of will and would.

One you will get the meaning correctly, "would" will stop being a "giant monster".

"I’d sit this one out, Cap," Natasha said.

My friend believed that it means :

"Natasha told Cap that she would not join the fight between Thor and
Iron Man, It's his (Cap) turn to do so."

I believe that this is a suggestion as :

"I would sit this one out if I were you, cap", Natasha said.

You are right and your friend is wrong. The sentence says absolutely nothing about what Natasha will or won't do. It says what she would do if she were Cap. But she is not Cap so she is not saying anything about herself.

Example

A world champion cage-fighter says to Bob who is not a fighter.

"I would sit this fight out Bob (if I were you) because you would not win. However if I fight I will win."