It is two weeks since I have seen him.( have seen-- saw)

It is two weeks since I have seen him.( have seen-- saw) it is three years since i have seen him 改错：It is 3 weeks since Ihave seen him. It is twenty years since I have seen her Since I have seen you i have seen it I have seen him yesterday. It is the best_____ I have seen. It has been a month since I have seen you. I have been here since I was born 和I haden't seen him since I saw him this morning.与I have seen him this morning 怎么翻译,区别 i never saw him和i have never seen him有什么区别?