Modal Verbs: Must, Have to, Need to

Modal verbs in English grammar express the ability, permission, obligation, possibility, etc. Modal verbs only have one form, meaning that they are used in simple present and don't have past simple and past participle forms.

However, some modal verbs can have alternative forms which can be used in different tenses.

Obligation

Rule 1: We can use must to express that something is necessary and give advice or an order in a strong way. Usually, such an obligation is imposed by the speaker.

Rule 2: We can use mustn't - contraction of must + not - to express that something is forbidden or not allowed.

Rule 3: We can use have to to express a general obligation (such as rules or laws) or an obligation in the past or future.

Rule 4: We can use need to to express that something is important for you to do, but rather at a given moment than as a reoccurring action (such as responsibility or duty).

Rule 5: We can use need not to and don't have to if there is no obligation at all. Don't have to is the contraction of do + not + have to.

Deduction

Rule 6: We can use must to express that something is true in the present, and we are 100% sure about it.

Rule 7: We can use must + have + past participle to speculate about something in the past.