提问,英语的一些单词、短语、句型用法

提问,英语的一些单词、短语、句型用法1、We are all here waiting for Miss Green to come 为什么用to come2、I saw a big dog lying under the tree when walking past the shop为什么不用was walking3、All has been done now.We can go.这里为什么用has?是All为不定代词还是将All看为一个整体还是什么?4、The students were all tired,but none of them stopped to have a rest-Do you want an apple,a banana or a pear?-None,they all look bad为什么用none,不是人和物吗?5、I think he must be in the office now,isn't he?是否情态动词反意疑问句都用be动词?