ENGLISH TENSES

I cannot don't understand English tenses. Can you help me?

Using "cannot" means that you may have a physical problem in your brain to understand things, in this case, tenses.  It can be used as an exaggeration which I don't see in your context.  "Don't" is better.

There are much differences in meaning with all these sentences...

You cannot understand.

You may not understand.

You do not understand.

You must not understand.

You should not understand.

You have not understood.

ENGLISH TENSES

I cannot understand english tenses. Can you help me?

No correction.  I can help with English tenses.  Do you have any specific examples to work with?