Step 1: General introduction
For reference:
1. If we talk about the ability someone has now, we use the modal verb can.
2. If we talk about the ability someone had in the past, we usually use the modal verb could.
3. If we want to talk about the ability someone will have in the future, we will use the form of the modal verb will /shall be able to.
4. Modal verbs should/ought to (sometimes have to) are always used to give advice or tell someone that he or she is under obligation to do something.
5. If we want to express that it is necessary for us to do something, modal verb must is often used.
6. Model verbs may/might, can/could are often used to ask for permission.
Examples:
