Modal Auxiliary Verbs - Must  

The modal verb must is most often used to express necessity—i.e., that something has to happen or be the case. We also use this sense of the word to indicate a strong intention to do something in the future, to emphasize something positive that we believe someone should do, and to rhetorically introduce or emphasize an opinion or sentiment. In addition to indicating necessity, must can be used to indicate that something is certain or very likely to happen or be true.

When must indicates that an action, circumstance, or situation is necessary, we usually use it in a declarative sentence. For example:

We can also use must in interrogative sentences to inquire whether something is necessary, usually as a criticism of some objectionable or undesirable action or behavior. For instance:

However, this usage is generally reserved for more formal speech and writing, and isn’t very common in everyday English.

We use the same meaning of must to indicate something we have a very strong intention of doing in the future. For example:

We also use this meaning to make suggestions to others of something positive we believe they should do, as in:

Finally, we can also use this meaning of must as a rhetorical device to politely introduce or emphasize an opinion or sentiment about something:

Note that we can accomplish the same thing by using the verbs let or allow instead, as in:

In addition to being used to indicate a necessary action or situation, must is also often used to describe that which is certain or extremely likely or probable to happen, occur, or be the case. For example:

Generally speaking, we do not use the negative of must (must not or mustn’t) to express a negative certainty or strong disbelief. Instead, we use cannot (often contracted as can’t), as in: