for后接时间段的名词词组，表示行为或状态持续了多久。
Mr. Li enjoyed the country life and lived there for almost fifteen years .
李先生喜欢乡村生活，并在乡下住了近15年。
She has worked there for four years .
她已经在那里工作四年了。
from后接时间点的名词词组，表示行为或状态的起始点，而不涉及其持续时间的长短。
He'll be here tomorrow from about seven o'clock onwards.
明天大约7点钟以后他在这里。
Students go to school from Monday to Friday.
学生们周一到周五上学。
since后接时间点的名词词组，不仅表示行为或状态的起始点，而且还强调该行为或状态从起始点一直持续到此时此刻，因而与延续性动词的现在完成时连用。
I have been translating the book since last year.
我从去年就开始翻译这本书。
The factory has been there since 1989.
那家工厂自1989年就在那里了。
She hasn't been home since her marriage .
她自结婚以后就未曾回过家。
