With Modals we normally need the Perfect to refer to something before now, but there are two meanings of can that are different:
When can refers to permission, the past form is could:
Could is also the past form of can when it refers to ability:
The present forms - can and be able to - have a very similar meaning.
But could and was able to are different:
It's not possible to use modal verbs with the perfect and future tenses, other modal verbs and used to: