English · Chapter 4 · Lesson 8 · Grammar primer
Modal verbs
The little helper verbs that change a verb's meaning - can, could, must, should, may and might, for ability, permission and obligation.
The short version
Mustn't and don't have to are opposites
Mustn't means the thing is forbidden. Don't have to means there is no obligation and the choice is yours. You mustn't come and You don't have to come say opposite things, so this is one to check rather than guess. Must usually comes from the speaker; have to comes from a rule or somebody else.
You mustn't tell anyone.
No -s, no to, no do with a modal
Modals share three rules that make them easier than ordinary verbs. They never take -s in the third person, the verb after them is the base form with no to, and they build their own questions and negatives without do. Ought to, have to and need to keep their to, because the to is part of the semi-modal.
She can swim.
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