In the world of English, modal verbs such as "can", "will", "shall", "may", "ought" and "must" help main verbs. For example, in the sentence, "he can run fast", the verb is "run" and the modal verb is "can".
Each modal verb helps other verbs to express different shades of meaning. "You can read the newspaper" is very different from "you must read the newspaper". MUST, in turn, is much stronger than OUGHT. "You ought to read the newspaper" is a recommendation.
Sometimes people ask permission by requesting, "Can I....?", to which pedantic people will always respond, "May you". CAN means "to be able" to do something, whereas MAY asks whether one is permitted to do something.
WILL is interesting because it is now used in English to create the future tense: "I will go to the cinema". In the past, WILL meant "to want", so "I will go to the cinema" would've meant "I want to go to the cinema" (not that there were cinemas so long ago...). Use modal verbs for precision in your writing.
To sharpen your modal verbs skills, have a go at the following quiz.