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）