How can I explain the difference between 'could' and 'can' when expressing possibility? Most of the time it is clear from the context, but use of can can also express possibility (rather than ability). For example, giving advice answering the question: How can/could I improve my English? You can/could listen to the radio, watch TV and read the newspaper. Both are possible. Students want to know when to use 'could' and when to use 'can'.