We use can and could to express the idea of ability in different contexts.
Can is the modal of ability for present situations. It is followed by the main verb:
Could is used to express the idea of ability in the past. It is also followed by the main verb:
be able to (optional)
Another way to express ability is by using be able to. Although it is not a modal, it can replace both can and could.
be able to (mandatory). In some situations, be able to is the only correct option.
After modal verbs (can, could, may, might, will, would, must, should).
When talking about one specific ocassion in the past:
The police were able to identify the criminal.
When talking about the future.
When using the perfect tenses (have + verb, had + verb).