At the outset, it seemed necessary to be on guard against the other two.

一开始，似乎有必要提防另外两个。

Two soldiers must be on guard all night.

两名战士必须通宵警戒。

I must protect myself and be on guard.

我必须保护自己、保持警惕。

Two soldiers must be on guard all night.

必须有两个士兵整夜站岗。

We should be on guard against such people.

我们应警惕这种人。