8. Can, could and be able to
The verb can is used to say that someone or something is able to do something. Can is called a modal verb. It doesn’t have all of the tenses that verbs usually have. It has the simple past tense could, but no past participle. When a past participle is needed, the expression be able to is used instead:
Can you lift that box?
I had no idea they could do things like that.
Since when have you been able to sing?
The negative forms of can and could are cannot and could not. In spoken and informal language, they are often shortened to can’t and couldn’t:
I can’t go out tonight.
They couldn’t hear anything.
Note that cannot is one word and could not consists of two separate words.