Expressing abilities with can, could and be able to

Use of can

http://www.englishclub.com/grammar/verbs-modals_can_1.htm

Use of could

http://www.englishclub.com/grammar/verbs-modals_can_2.htm

Use of be able to

http://www.englishclub.com/grammar/verbs-modals_can_3.htm

Look at the examples and explanations about the use can, could and be able to and do the quiz
http://www.grammar-quizzes.com/modal4.html