Must: adverb, emphasizing mood, often used as adverbial.
(a) Express necessity and emphasize facts or necessity. For example, "You must show courage." "We must be modest and prudent and avoid jealousy." (1) is followed by a verb, and (2) is followed by an adjective. Sometimes it can also be used before clauses, such as: "If you want to make rapid progress, you must work together with Qi Xin."
(2) Strengthen the tone of the command. For example, "You have to handle this alone." He must go to the hospital for a serious examination.
(3) The negative form of "must" is "unnecessary, unnecessary, unnecessary."
The verb "necessary" means that there must be something indispensable. As an attribute or predicate. For example, "as the price of raw materials necessary for production rises, the production cost is also rising." Air and water are necessary for everyone. "