These
modal verbs are used to express the ability or possibility that someone has to perform an action. These verbs are '
can', '
could', '
can't', '
couldn't' and '
be able to'.
Samuel can speak four languages.
I wasn't able to finish my exam.
'Can' is used for actions that occur in the present or the future, whereas 'could' is used for actions that occur in the past.
In addition, 'can' and 'could' are also used for requests and to ask for permission. In these cases, could is a bit more formal than can.
Present:
can / can't (for both general and specific abilities):
She can speak English.
He can't drive – he's too tired.
We can't come now.
Past:
could / couldn't (for general ability):
I could read when I was four.
He couldn't dance at all until he took lessons.
My grandfather couldn't swim.