"Can have" vs. "could have"

Can have implies that someone has the opportunity to possess or do something.

Edward can have an ice cream cone when he gets home.

Gillian can have her friends over after school.

I can have a whole bottle of wine with my dinner if I so desire.

It implies permission or ability.

Could have implies the same thing except that the opportunity is not immediate or is based on a contingency, or that the statement is contrary to actual fact.

Edward could have an ice cream cone when he gets home, but he'll have to finish his homework first.

Gillian could have her friends over after school if she wasn't so busy.

I could have a whole bottle of wine with my dinner but for the fact that I don't want to get caught driving drunk.

It is also used in the past tense to express a possibility that no longer exists.

I could have had it all — a wife, a home, a family — but I blew all that when I became an alcoholic.

Believe it or not, there actually is a slight difference between the two:

He can have the hammer. (He has the necessary permission to possess the hammer.)

Some people consider this wrong and that the correct word is may, but in contemporary English, using can have signifies permission over ability, because the cases where someone is unable to possess something are very few and far between. This is not necessarily true in other cases—"You can go the bathroom", as drilled in by so many teachers, does express ability.

He could have the hammer. (He has the potential to have the hammer already in his possession.)

The speaker does not know who has the hammer, but he has the potential to have it.

If I take the large box, he can/could have the hammer lying underneath it.

This is one of the few cases I mentioned above where he might not actually be able to possess the hammer. In this case, can signifies ability, because presumably, the box is on top of the hammer, thus not allowing him to reach it. Even here, there is a slight difference—using the word could over can could, depending on the audience, sound either formal or reluctant.

The original poster asked specifically about "certain past forms" and the answers thus provided both address the present tense.

I have a feeling he was referring to past modal constructions like:

Where is he?
I don't know; he could have gotten stuck in a traffic jam.

That road sign was in Spanish, but we've only been driving for an hour. We can't have already crossed the border!

As far as I know, the construction can have is never used to refer to past possibilities, but I can't speak with any authority about British English, because I'm from the States. However, because the final 'T' of "can't" is often replaced with a glottal stop in both American and British dialects, it's possible that the original poster was simply mishearing can't have as can have.

See Modal Verbs of Probability.