英语的现在完成时的since和for的区别，4个小时，和4个小时前分别用什么

你好，
1) since +过去一个时间点　(如具体的年、月、日期、钟点、1980,last month,half past six).
I have been here since 1989.
2) since +一段时间+ ago
I have been here since five months ago.
3) since +从句　
Great changes have taken place since you left.
Considerable time has elapsed since we have been here.
4) It is +一段时间+ since从句
It is two years since I became a postgraduate student.
since的用法主要有两点：一是表示“自从……以来”,二是表示“既然”之意.但如果只了解词的含义,而不了解其用法,在使用时就会造成错误.例如：I have not heard from him since I lived in Shanghai.
for +时间段，for“累计、达”,后面跟时间段,不可以跟时间点.
所以是，
for four hours
since four hours ago
希望对你有帮助~

since +时间点
for +时间段

for four hours
since four hours ago