YouTube study knowledge Facebook page Modal Auxiliaries Modal auxiliaries help to express ability,possibility,permission and obligation. They do not change with the person or number of the subject. Function Can- Can is used- (1) to express ability. -He can play the guitar (2) to express or ask for permission -You can turn in your assignment next week. (3) to express possibility -Such things can happen if you are not careful. Could- Could is used- (1) to express ability in the past - I could walk ten kilometer as a young man (2) to express conditional abili...