'Must' is a modal verb (also known as modals). Modals give additional information about the main verb of the sentence. 'Must' is used to express necessities, possibilities, and suggestions. Have a look:

'Need' is a semi-modal verb (also known as semi-modals). It can function as both a modal and a main verb. When used as a main verb, 'need' is followed by a 'to-infinitive', a gerund, or a noun phrase, while the modal 'need' is followed by a bare infinitive and is mostly used in negative and interrogative sentences. Semi-modal 'need' has the same form for all tenses, genders, and numbers. For example:

Both 'must' and 'need' are used to talk about a sense of necessity or obligation. However, 'must' is used to express a strong sense of obligation or necessity. It suggests that something is required or necessary and that there is no other option. For example:

'Need', on the other hand, is used to express a less strong or urgent sense of necessity. It suggests that something would be helpful or beneficial, but that it is not strictly required or necessary. For example:

'Need' and 'must' can both be used in negative forms and the structures of their negative forms are similar.

The negative form of 'must,' 'must not' or 'mustn't' is used to express prohibition. However, it is relatively uncommon and may not be heard frequently in everyday conversation.

'Needn't' or 'need not' is commonly used to indicate that something is not necessary or required. This construction is frequently used to express the absence of obligation or necessity.

'Must' and 'need' can both be used to form questions. To create questions with 'must' or 'need,' we invert the subject and the modal verb. For example: