We use can and could to express the idea of ability in different contexts.

Can

Can is the modal of ability for present situations. It is followed by the main verb:

• I can speak many languages.

Could

Could is used to express the idea of ability in the past. It is also followed by the main verb:

• My grandfather could dance salsa when he was young.

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.

• She is able to ride her bike with her eyes closed. (She can ride her bike with her eyes closed.)
• He was able to drive well. *(He could drive well.)

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).

  • I think he will be able to finish the work without help. I think he will can finish the work without help.
  • She might be able to come tonight. She might can come tonight.

• When talking about one specific ocassion in the past:

• The police were able to identify the criminal.

• When talking about the future.

  • You are not going to be able to go because you have promised to help.

• When using the perfect tenses (have + verb, had + verb).

  • I haven’t been able to go to the gym recently.