The modal verb can has two tense forms of the indicative mood: the present tense can and the past tense could.
- Affirmative (+) form (I can, I could)
- Negative (−) form (I can’t, cannot))
- Question (?) form (Can I borrow your pen?)
The verb can has the following meanings:
1. Mental, physical or circumstantial ability to do something.
- She can do sums in her head
- He can lift this suitcase
Note: When could is used with reference to the past, it denotes only the ability or possibility of performing an action but not the realisation of the action.
To indicate the achievement in the past we use was/were able to do smth, managed to do sth or succeeded in doing sth (the latter is used in literary style):
- I’m happy I was able to help you
- They will be able to do it next year.
If an action was not realised in a particular situation in the past it is expressed with the help of failed to do smth, wasn’t able to do smth, didn’t manage to do smth, didn’t succeed in doing.
- We didn’t succeed in translating the text
- You can take my book
- Can I go out?
- Could you help us?
4. Prohibition (informal)
- You can’t cross the street here
5. Uncertainty, strong doubt, astonishment (in interroga¬tive sentences). In these meanings can is used with all forms of 6 the infinitive. Could expresses greater doubt. Thus, the time-reference is indicated by the form of the infinitive.
- Can/Could it be true?
Can with a perfect continuous infinitive expresses actions which began in the past and continued up or into the present moment:
- Can (could) she have been talking on the telephone so long?
6. Improbability, strong doubt (in negative sentences)
- It can’t/couldn’t be true