由时间状语来判定
一般说来，各种时态都有特定的时间状语。与过去完成时连用的时间状语有：
（ 1 ） by + 过去的时间点。
如： I had finished reading the novel by nine oclock last night.
（ 2 ） by the end of + 过去的时间点。
如： We had learned over two thousand English words by the end of last term.
（ 3 ） before + 过去的时间点。
如： They had planted six hundred trees before last Wednesday.