一个虚拟语气句子：
If there was no gravity, we should not be able to walk，该句是错的，应该写成:
If there were no gravity, we should not be able to walk.
还有同种情况的句子：If I were rich, I should help the poor.
If my theacher were here, he might be able to help.
但是问题来了，下面这个句子：
If it weren't raining, I would not stay in the classroom.又是错的，应该写成：
If it wasn't raining, I would not stay in the classroom.解释是：此句中be为助动词，而非be动词。
希望高手帮我解答一下这个句子，另外说一下be动词和助动词如何区分，谢谢
最后一句还真是助动词，刚才突然想明白一点了。请高手给说说怎么能比较简单地判断出来be动词还是助动词，目前这两个答复我实在不能采纳为答案。