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Hi again

I just want to make sure that I understand this properly.

Here are some examples:

I could've done better.(but I didn't)- I've always understood this.
I couldn't have done that without you. - What if I use just couldn't?
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Comments  
Dusan StojilkovicI could've done better.(but I didn't)
I had the ability to do better. (but I didn't)
Dusan StojilkovicI couldn't have done that without you.
It would not have been possible for me to do that without your help. (but it was possible because you helped)

This is a more of less standard formulaic expression of gratitude for someone's help.
Dusan StojilkovicWhat if I use just couldn't?
I couldn't do that without you. (It sounds a little strange to me.)

~ I would not be able to do that (in the future) without your help.
OR
~ I failed in my attempts to do that because I did not have your help.

CJ
So, should I always use couldn't have when I want to talk about every possibility in the past?
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Dusan StojilkovicSo, should I always use couldn't have when I want to talk about every possibility in the past?
I can't answer "yes" because of the underlined words above. There are always exceptions.

I think a better procedure would be to post a few sentences with "could have" and/or "couldn't have" that you think follow this rule that you've come up with.

Note also that "may have" and "might have" can also be used to express past possibilities.

CJ
Now I think I understand this perfectly.

Anyway here are some sentences:

I could've danced all night. ( But I didn't dance all night)
I couldn't have danced all night. ( I didn't have ability, so I didn't do it)

He might've left some papers on the table. ( The papers might be on the table, might not)

Am I right about this?
Dusan StojilkovicHe might've left some papers on the table. ( The papers might be on the table, might not)
It is possible that he left some papers on the table. Yes. So they might be there now - or might not.
Dusan StojilkovicI could've danced all night. ( But I didn't dance all night)
I couldn't have danced all night. ( I didn't have ability, so I didn't do it)
The problem with 'could' is that it often has an ability reading and a possibility reading.

I could have : I had the ability / would have had the ability OR It is logically possible that I did
I could not have: I did not have the ability / would not have had the ability OR it is not logically possible that I did

I could have danced all night. ~ I had the ability (desire, ...) to dance all night.
I could have danced all night. ~ It is logically possible that I danced all night. / Maybe I danced all night. (I don't know because I was so drugged or drunk.)

I couldn't have danced all night. ~ I did not have the ability (stamina, energy) to dance all night.
I couldn't have danced all night. ~ It is not logically possible that I danced all night. (I was in the hospital because of a broken leg.)

CJ
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So, basically could have means that I had ability.

But when I say: I could have danced all night.- The first thing that come up on my mind is the missed opportunity in the past.
Jim, when I want to say that I didn't do something in the past can say like this: I could've screamed yesterday. - I missed the opportunity in the past. Do I have to say: I could've screamed yesterday, but I didn't.

?
Dusan Stojilkovicwhen I want to say that I didn't do something in the past
Just say I didn't do it.

To focus on the fact that you had the ability to do so, but did not use your ability to do so:

I could have (done it), but I didn't.

The scenario is a bit unusual for talking about missed opportunities. Let's try something different for that.

The famous author Salvatore Puntkovski was at the local bookstore yesterday afternoon. I could've gone there and had him sign my copy of his latest novel, but I missed the opportunity because I had to go to work.

I don't know if that answers your concerns.

CJ
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