The document discusses the differences between using "can" and "be able to" when talking about ability in English. It states that "can" is used in the present tense to talk about ability, while "could" is used for past ability. "Be able to" is more formal than "can" in the present, but is necessary to talk about specific past successes or failures, rather than general past ability. Both "could" and "be able to" can be used to talk about not being capable of something in the past.