What is the difference between modals and the subjunctive/imperative mood?
Modal verbs are words used together with other verbs to express things like possibility, ability, obligation and so on.
The boy could read when he was five.
You must return the book to the library tomorrow.
May I have a cup of tea, please?
The sentence moods are forms showing speaker's attitude to the sentence. They can be:
indicative - the sentence gives an information: I like football.
subjunctive - gives an assumption on a hypothetical situation:
If I had more money, I would buy a larger house.
imperative - gives an order or command:
Return the book to the library before Friday.