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I am very confused with usage of could and could have +p.p..
For example, do these sentences mean the same:

  1. I couldn't come to college last week.
  2. I couldn't have come to college last week.
  3. I couldn't have come to college last week because I was working.

I know that could refers to ability to do something, while could have + p.p refers to some possibility in the past. However, in the examples above there is no difference because they bear the same meaning to me:

  1. I was not able to come to college last week.
  2. I was not able to come to college last week because I was working.

Another confusing examples would be:

  1. This book could not be written without your help. (Here I'm speaking about the past, not about the future.)
  2. This book could not have been written without your help.

Thank you very much!

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"couldn't" is only used to refer to actions that were not performed because of impossibility or inability in the past.

I couldn't come to college last week (even if I wanted to).

means "I was not able to come to college last week" or "It was not possible for me to come to college last week."

Instead, "couldn't have" can express other meanings. I will use some other examples for you to understand the meaning and then your own example explained:

  • Inference or deduction of past impossibility:

• George Washington could not have known Abraham Lincoln – they lived at different times.

• As a young woman who had never given birth before, she couldn't have known what the experience of childbirth would be like.

You say you saw me in college last week but I couldn't have come to college because I was abroad last week.

  • Past conditionals:

• The team couldn't have won the game if they had not trained so intensively.

I couldn't have come to college if I hadn't given me a ride. (But you gave me a ride, and I was able to attend.)

In addition to its use in past hypothetical conditional sentences complete with a main and a conditional clause, "couldn’t have" is also found in incomplete conditionals in which the if- clause is merely implied, for example:

• I couldn't have done it without my family [if I hadn't had my family].

• The timing couldn't have been worse [even if circumstances had been different].

• I couldn't have written a better essay [even if I'd tried to do it].

I couldn't have come to college [even if I had wanted to].

With actions that involve some ability or skill, "couldn’t have" expresses past hypothetical inability, similar to wouldn't have been able to do sth or wouldn't have been capable of doing sth:

• I wish I had written Sunset Park, but I couldn't have, because no one can write like Auster.

As regards your last two examples:

"This book could not be written without your help" means that it was impossible to write the book because the other person did not offer or provide his/her help and, as a result, the book WAS NOT written.

"This book could not have been written without your help" means that, thanks to the other person's help, it was possible to write the book, so the book WAS written.

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In addition to Gustavson's excellent answer, here's a bit more nuance around "couldn't" and "couldn't have"...

If I heard someone say this:

I couldn't come to college last week because I was working.

...then the implication is that they made a choice between work and college, and chose work.

If I heard someone say this instead:

I couldn't have come to college last week because I was working.

...then the implication is that they are using work as an excuse for not going to college.

That said, this is a subtle distinction, and the context matters. If I heard someone say this:

I couldn't come to college because of the tornado.

...then of course, they didn't choose for the tornado to happen. So in that case, "couldn't" and "couldn't have" are equivalent; of course you had no choice, because a tornado is outside your control. (Well, except for students of certain schools.)

Using "couldn't have" for something that is technically a choice - like going to work instead of college - implies that going to work is a necessity; it implies that you don't really have a choice at all. Your audience may or may not find this believable.

  • That was indeed a nice addition. – Gustavson Mar 3 '17 at 19:38

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