Modal Verbs: Can/Could

Modal verbs in English grammar express the ability, permission, obligation, possibility, etc. Modal verbs only have one form, meaning that they are used in simple present and don't have past simple and past participle forms.

However, some modal verbs can have alternative forms which can be used in different tenses.

Ability

Rule 1: We use can to express ability only in the simple present tense.

Rule 2: We can use an alternative form to be able to to express ability in all tenses (present, past, perfect, continuous, and future).

Rule 3: We can use an alternative form could to express the general ability and with the verbs feel, hear, see, smell, taste, remember and understand in past simple.

Rule 4: We can use an alternative form was/were able to to talk about something specific in the past simple.

Permission

Rule 5: We can use both can and could to ask permission in present simple. Could is more polite than can.

Rule 6: We can only use can to give permission.

Rule 7: We use could in conditional clauses and in the simple past.

Negatives

Rule 8: We use can't (a short form of cannot) to express something we are sure is untrue or impossible in the present.

Rule 9: We use can’t + have + past participle to speculate about something in the past.

Rule 10: We use both couldn’t (could + not) and wasn’t/weren’t able to (was + not/were + not able to) to create negative forms.