2

I was trying to solve a physics problem, but it was a bit too difficult. What sounds natural:

I wasn't able to do this question. (Or "solve") [Or maybe: "I was unable to do this question"]

I couldn't do this question.

What is a native more likely to use "wasn't able to" or "couldn't"?

3

Either are perfectly acceptable - in terms of wasn't able vs couldn't. I have heard both said in the past and wouldn't think either sounded unnatural.

On a side note, unless you have the exam in your hand and pointing at a particular question, I would change it to:

I wasn't able to/couldn't do that question

For example, you're chatting after an exam:

Joe: What did you guys think of the first question; the one about gravitational forces?

Mary: I couldn't do that question at all.

Bob: Yeah, I wasn’t able to do it either.

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2

They both make perfect sense, and each is normal and acceptable.

The one consideration I can think of that would distinguish the two is simply the fact that I couldn't is only two words as opposed to the four in I wasn't able to.

Also, I couldn't is easier to pronounce than I wasn't, given the movement of the tongue required for each.

As such, the I couldn't version is more likely to actually be spoken because it's shorter and simpler, something, everything being equal, we tend to choose. (But perhaps in formal writing, the longer version would have an edge.)

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0

I would be inclined to use "solve" (particularly for a physics or math problem) or "answer" rather than the generic "do" as a verb.

  • I wasn't able to solve problem 5.
  • I couldn't solve problem 5.
  • I wasn't able to answer the question about Brahms.
  • I couldn't answer the question about Brahms.

All of the above would feel quite natural to me.

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