If you really want yourself to be in good health,you must---

这道题你的问题在于情态动词的用法没掌握好。情态动词有can (could),may (might),must,have to,shall (should,will (would),dare (dared),need (needed),ought to等。它们的用法就是：肯定：情态动词 + be否定：情态动词 + not be此题只是使用了情态动词的否定用法和动词ing表状态延续的用法。