The difference between "must" and "necessary":
"Must" generally means that it is necessary to deal with the matter, and it is more or less with the tone of command, that is, it must mean. It's a good idea to have a good time, but it's not a bad idea to have a good time. For example, food is a necessary food for human beings. Toothbrush and toothpaste are necessary daily necessities.
"Must" refers to work and things that must be done, and it is a willing verb, often combined with other verbs. It is an imperative verb, often combined with other verbs. And "necessary" refers to the raw materials, goods, uses and other essential needs, it is a verb, can be used alone, can also be used as a determiner, such as: necessary daily necessities in the necessary is the determiner.
The words "须" and "需" have the same sound but different meanings. The original meaning of "need" is to stop and wait in case of rain, and the common meaning is "need", but now it is not used alone, and can only be used to form synthetic words or fixed phrases, which shows the usage of verbs (e.g., "to need, to require, to need, to take what one needs").
Expanded information:
Necessity, indicating necessity of matter and necessity of sense, adverb, with emphasized tone, mostly used as gerund. Must be in the application of often confused with "necessary", "must" and "necessary" the same sound and meaning, both have the meaning of necessary, the difference is that the former emphasizes the necessary, the latter emphasizes the necessary have.
Necessary, meaning must have; can not be less. Easily confused with must.
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