must，作实义动词和情态动词的区别

一、实义动词，有具体的词汇意义，表示主语发出的动作或所处的状态。在陈述句中，实义动词可以单独作谓语。实义动词加表语（主补）、宾语或状语构成谓语部分。

She is an English teacher and she works hard. (单词动词作谓语)

He enjoys swimming and gave us a performance last Sunday. (单词动词作谓语)

Mr Smith has been teaching in a middle school for 30 years. (加助动词作谓语)

（二）情态助动词具有不同的情态意义。（will会，将， would会，将 shall将， should应该 can可能，也许， could可能 may也许， might可能 muts必须，一定， have / had to不得不 used to过去常常， need需要， dare敢， ought to应该...）

如：I haven't received his letter for a whole month. (加haven't构成完成时态)

Something must be done to make the rivers clean.