Modals of Ability
Present Can + (V) Can’t/Cannot + (V)
= am/is/are able to + (V)
I can finish my assignment soon.
I can’t understand this Maths problem.
Past Could + (V) (general ability)
Was/were able to + (V) (on a particular occasion)
I always thought I could understand Maths, but yesterday I was not able to work out what the teacher meant.
Negative Couldn’t + (V) (both for general ability and for particular occasions)
I couldn’t understand the Physics teacher all last year.
Yesterday I couldn’t understand this year’s teacher, although I generally can.
All other tenses Use: be able to + (V)
I have been able to keep up with my studies so far.
I hope I will be able to pass these exams.