There在现在完成时中是There have还是has

There在现在完成时中是There have还是has 英语句子中各种时态分别有些什么特定词,比如比如 现在完成时是there be现在完成进行是there have been there's none left.中left是leave的过去分词,那这一句是现在完成时吗?为什么不用加have/has 现在完成时中 never的问题I never have been there行不行?还是一定要I have never been there There __ few students in the library after school1.has been2.have been3.are在题中,是否用现在完成时,强调一直以来的状态呢?该如何选? 关于英语中 have been to 的用法I have been there for six months.这句话是现在完成时,there是地点副词前不加介词.所以have been to 表示曾经去过某地,现在已经不在那了.但是我看书本上翻译说“我已经在 since 用在一般过去时和现在完成时中的区别He has worked there since 1990.He worked there since 1990.这两句话中 都用了since .两句话的意思有什么区别? 关于英语语法和时态几个跟旅游有关的常用句型：(1) I have (never) been to.../here/there before.Have you ever been to.../here/ there before?这个been在句子中算什么时态,是被动语态吗,还是现在完成时,为什么有 The dog was trapped there for two weeks 后面的for two weeks 不是用在现在完成时吗 前面咋是was There is...中there是副词还是指示代词 英语中there能做主语吗?能分析一下there is nobody there的句子结构吗?我在书上看到there is nobody there中的nobody是做表语的！那么它是特殊情况还是书上解释有误？ there be句型中后面是不可数名词复数,前面是there is还是there are? There are many people there who speak there 和who在这个句子中又是什么意思? There there there there [there