be应该怎么用？

be 是am is are的原形
就好象was 和were 是它们的过去式一样

规定过去发生的事情要用过去式,也规定will would shall must can 和 动词不定式等等 后面要用动词原形

举个简单例子
He is a student
He must be a student.

不要让这困扰你,多看看就习惯了.

be
[bi:]
vi
现在时 I am, you are, he is, we are, you are, they are （缩略式 I'm, you're, he's, we're, you're, they're）, （否定缩略式 I'm not, isn't, aren't）, 过去时 I was, you were, he was, we were, you were, they were （过去时否定缩略式 wasn't, weren't）, 过去分词 been, 现在分词 being

将来时的构成只有两种形式：
1．will be +动词原形
2．be going to do sth.

另外，关于there be 结构的将来时构成为：
1．there will be
2.there be going to do

以上是最简单易懂的，希望对你有帮助＾0＾