We use CAN to express ABILITY in the PRESENT. Example: He can speak three languages. She can´t dance.

Ask for premission in a formal way. Example: Can I go to the bathroom?

We use COULD to express ability in the PAST. Example: He could speak three languajes. She couldn´t drink beer

We use SHOULD to ask or give advice. Example: I feel sick, What "Should" I do? R= You should go to the doctor. You shouldn´t eat sweets.

We use MUST for a total OBLIGATION. Example:We must do homework. We must not fail the exam.

For a very certain PROBABILITY. Example: He must dance in a performance.

We use OUGHT TO for a partial OBLIGATION. Example: We ought to wash the dishes. We ought not to stay up late.

We use MAY to ask a permission. Example: May I take photos?

For a not so sure PROBABILITY. Example: They may have problems.