Both need and must mean that something should or has to happen. How you use them and what exactly they mean when you do, however, is slightly different.
Things that you need are typically either verbs or things. With verbs, the word need should be followed by to. For example:
- I need to do my homework by the end of today.
- My mother needs me to help her with the dishes. (Note that if you need someone else to do the action, the person is used as an object in the sentence.)
- Jack needs more sugar for his cake recipe.
In each of these instances, the person in the sentence needs either an action to be done or a thing that can help them complete an action.
By contrast, must has more potential uses. It does not need the word to regardless of what comes after it. It addition, it is always followed by a verb. Even if the thing that you need is a noun (sugar for your cake, for example), if you use the word must you need to follow it with a noun (must have more sugar). Consider the following examples:
- The family must go to Austin to claim their property.
- Justin must have studied a lot to get such a high score on his test.
Both of these examples use must to express necessity. However, must can also show that something is either likely to happen, emphasize something, or communicate a desire for something to happen. Take note of these examples:
- He must really like to read. (The probability of him liking to read is very high.)
- I must ask, what did you think of the movie? (The emphasis is on the shoes.)
- The next time that you’re in town, you must stop by our house! (The speaker wants the person they are talking to, to pay them a visit.)
When asking a question, must can express some disappointment or frustration. This is especially true if the thing that happens is because of luck.
- Must the train be late on today, when I have a job interview, of all days?
- Why must you be here when this meeting has nothing to do with you?
Lastly, must is more formal than need or need to. If you must do something, that is better for situations in which you are talking to a superior.
- I must request that you stay. (This is more formal than “I need to request that you stay” or “I have to request that you stay”.)
