现在完成时

　　1.现在完成时用来表示过去开始，但持续到现在的动作，或表示过去发生的动作，但对现在仍留下某种后果和影响。常和already , before , in the past few months , over the past 5 years, just , so far , up to now ,yet 等状语连用。

　　He can't go on holiday because He has broken his leg . 因为他把腿摔伤了，所以不能去度假。

　　I haven't seen her since 1991. 自1991年以来我就没有再看见她。

　　2.在It is the first time + that – 分句结构或It / This is +形容词最高级+名词后的从句中用现在完成时

　　It's the third time I have explained this particular aspect of grammar. 这是我第三次讲解这条语法的特殊用法。

　　He's the most interesting person I have ever met .