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We use "was able to / couldn't (for specific ability in the past)"

Ex: I couldn't pass the exam yesterday

But then, we use "Couldn't have + past participle to mean that something wasn't possible in the past, even if you had wanted to do it."

Ex: I couldn't have passed the exam yesterday

So, What are the differences between "I couldn't pass the exam" & "I couldn't have passed the exam"?

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I couldn't pass the exam.

When you took the exam you failed it.

OR

You have no confidence that you could pass the exam at a future date.

I couldn't have passed the exam.

You didn't take the exam, but you would have failed it if you did.

OR

You did take the exam and want to express the hopelessness of your situation: that you didn't pass and there was no way you could have passed.

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  • would it have completely different meaning if I said it in a way " I could not have passed the exam if I did not look at notes you gave in last minute." What I mean is that the student passed the exam but thinks it would not possible if the notes were available. – Mrt Feb 4 '17 at 3:42
  • @Robusto, I think you are talking about "Could Have" as Probability (a guess). So, "I couldn't have passed the exam." = "I'm guessing that I couldn't pass the exam because it was so hard" (You did not get the result at the time you said, so you didn't believe in yourself). But "Could Have" can be expressed as unreal ability. " I could not have passed the exam if I did not look at notes you gave in last minute."= I actually passed the exam, I got the result before I said that. – Tom Feb 4 '17 at 4:40
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    @Mrt: Yes, of course. If you change the sentence to add a condition or a contingency, that changes it. As it would change any sentence. – Robusto Feb 4 '17 at 4:56
  • @Tom: I'm talking about what those sentences mean in the preponderance of use cases. – Robusto Feb 4 '17 at 4:57
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Both sentences are different in meaning.

The former is a complete sentence that indicates your failure in the exam. On the other hand, the latter isn't a complete sentence; it's the main clause of a hypothetical sentence that also has a subordinating if-clause. For example:

If he hadn't helped me, I couldn't have passed the exam.

The sentence shows hypothetical or nonfactual negative cause and effect. In fact, it indicates that he helped you and you passed the exam.

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  • But "Could Have" can be used to express Probability (a guess). "He could have been late" = "I would guess He was late, but I am not so sure." – Tom Feb 4 '17 at 4:50
  • It refers to an impossible or imaginary situation in the past. – Khan Feb 4 '17 at 5:46
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I could not pass the exam.

This is either a simple statement that the speaker was unable to pass the exam in the past, or that the speaker contemplates being unable to pass the exam in the future.

Why aren't you practicing law nowadays?
-- I tried, but I could not pass the bar exam.

Why don't you consider becoming a lawyer?
-- I could not pass the bar exam.

I could not have passed the exam.

This is a more complicated statement, which refers to the past inability to pass the exam, but as viewed in hindsight in relation to a counterfactual circumstance. For example:

I had had no sleep the night before the exam because the people living in the room above me were partying all night. So I could not have passed that exam no matter how well I knew the subject matter. I was falling asleep at my desk!

The phrase no matter how well I knew the subject matter refers to a counterfactual situation, one where the speaker postulates having perhaps even an expert's knowledge of the material. Even if he had been an expert on the subject, he could not have passed the exam because of his lack of sleep the night before.

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  • In your second example, did he know his failed result at the time he said "I could not have passed that exam"? If he did, then he would say "I could not pass the exam" right?. – Tom Feb 4 '17 at 15:54
  • The choice between simple past and the perfect version does not hinge upon his knowledge at the time. He could have known, while taking the test, that there was no way on earth that he was going to pass, because he felt so sleepy. Looking back upon that episode, he can remark "I could not have passed that test under any circumstances, considering how sleepy I was at the time. And I knew at the time that I wasn't going to pass. I was like Salvador Dali hallucinating over an empty teacup." – Tᴚoɯɐuo Feb 4 '17 at 17:34

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