现在完成时 have/has done过去完成时had done一个句子如果有Since（不做原因这个意思），yet,already等词，一般这是一个完成时态的句子，用现在还是过去就要看情况定了。Eg. I haven't finished my homework yet.
I have already been there for several times.
I had lived here since 1999.(也就是说说话的这个人是从1999年开始住在这个地方，而现在很有可能要搬走了）
