The document discusses the uses of the modal verbs CAN, COULD, and BE ABLE TO. CAN is used to talk about ability, possibility, and permission in the present and future. COULD is the past form of CAN and is used to talk about general ability or possibility in the past. BE ABLE TO is used for all other tenses besides the present and can be used in the past to sound more formal. COULD and COULDN'T are used for requests and talking about specific instances of ability in the past, while CAN'T expresses certainty that something is impossible or untrue in the present.