即完成时+时间段/次数前可用for也可不用for ,如：We’ve been here (for) three weeks/times .但
1)否定句中,for不能省.如：
I haven’t seen him for ten years .
2）时间状语位于句首,不能省for.如：
For the whole summer ,the students have worked on the same project.
3)固定搭配或模糊的时间段前不能省略,如：for a long time,for years等.
注意时间点前一定要用since,不能省略,即：since 3 days ago.
完成时态+(for)+时间段和+since+时间点主要注意动词一定是延续性动词,如不是,必须得换成延续性的动词或be+形容词.经典句是：My dog has been dead (for )3 days.而不能是My dog has dead (for )3 days.
下面列出常用瞬时动词的转延时的办法：
die-be dead,buy/receive/get-keep/have,leave-be away,come-be here/be in,go out-be out,begin/start- be on,move to-live in,borrow-keep,finish/end-be over,join-be in/be amember of,open-be open/keep it open,fall asleep/fall ill-be asleep/be ill,get up-be up,wake up-be awake,catch a cold-have a cold,go to sleep-sleep,put on-wear,meet-be together,make friends-be friends,go to school- be a student等.
