现在完成时是指过去发生的事情对现在有影响.
过去完成时就是把现在完成时提前一个时态,就是过去的过去.
例子I have studied English for 3 years.( 从过去持续到现在)
Mr Green had lived in New York for ten years before he came to China.("来”中国已过去,而先前“住”在纽约更过去!)