"Must," means, "have to do." For example, I could say, "I have to do my homework," or "I must do my homework." They mean the exact same thing. There are a few other things to remember about "must." It does not and an "-s," ever. "He must go do the store." You cannot says, "He musts...." Also, it seems that using "must," makes the sentence sound more urgent and important. Yes, you used, "in," correctly in that sentence. ps:(Does the preposition(in) used correctly in this sentence.or it should use on、at?) The sentence does have some other problems, though. It should be, "Is the preposition "in," used correctly in this sentence or should it be "on," or, "at"?" You need to use the verb "to be," in this sentence because the past participle of "to use" is being used as an adjective.

Elvis. "Must" in English is closest to "必須" (or perhaps "務必") in Chinese, and the usage very closely matches. To add to what Eric said above, "have to" is probably more commonly used than "must" in everyday speech. But I agree, they are nearly interchangeable.