Probability: Differences in the usage of can/could/may/might: May/might/could: to talk about future probability: There may/might be a downturn in the market You can use could though it expresses a less definite idea Can: to talk about theoretical or general possibility: I don´t think this ointment can relieve sunburn effects Could: to talk about theoretical or general possibility in the past: Most people thought that anything could happen Can: to talk about choices or suggest opportunities: After this bad result we can either change our strategy or we can look for an expert Can: We use can in questions and negative sentences to talk about the logical possibility that something is true: Look! The light in John`s room is on, but John is abroad. Who can it be? It can`t be John May/might/could: talk about the logical possibility that something is true in affirmative sentences: Where is John? He could/may/might be at Claire`s house May/might not: with the meaning of Perhaps: He may not work here any longer (It is possible) Can/could not: with the meaning of Certainly: He can/could not work here any longer (It is not possible)