must，作實義動詞和情態動詞的區別

一、實義動詞，有具體的詞彙意義，表示主語發出的動作或所處的狀態。在陳述句中，實義動詞可以單獨作謂語。實義動詞加表語（主補）、賓語或狀語構成謂語部分。

She is an English teacher and she works hard. (單詞動詞作謂語)

He enjoys swimming and gave us a performance last Sunday. (單詞動詞作謂語)

Mr Smith has been teaching in a middle school for 30 years. (加助動詞作謂語)

（二）情態助動詞具有不同的情態意義。（will會，將， would會，將 shall將， should應該 can可能，也許， could可能 may也許， might可能 muts必須，一定， have / had to不得不 used to過去常常， need需要， dare敢， ought to應該...）

如：I haven't received his letter for a whole month. (加haven't構成完成時態)

Something must be done to make the rivers clean.