时态，语气，格

现在：<现>  
过去：<过>  
将来：will <现>

一般：<现>  
进行：be <现分>   
完成：have <过分>

被动：be <过分>

虚拟语气：if+
　现：过去一般；过去(were)将来一般
　过：过去完成；过去将来完成
　将：过去将来(should)一般；过去将来一般
愿望：wish+
　现：过去一般
　过：过去完成
　将：过去将来(would,could,might)一般

从句时态：主将从现，主过从过，态看意思和连接词

原则：时态根据动词表示的时间

现完标志词：up to now/ so far/ recently/ lately/ in recent years

have done sth. three times
For the first time I did sth.,…
the first time I did sth
It was the first time that I had done sth.

since:

上下文时态一致用before, 不一致since

who/whom:

1. Mr. Smith introduced a young scientist _____ （he said）插入语 he had met at an international conference.
   A. who B. which C. whom D. whose

定语从句，修饰先行词scientist。先行词在从句中作谓语动词had met后面的宾语，用whom