这两个词都可以和表示时间的词语连用。一般说来，for后接一段时间，用以说明某事一直进行了多长时间，而since后接某一时间(过去的一个时间点)，用以表示某件事开始的时间。
since 的四种用法
1.since+过去一个时间点（如具体的年、月、日期、钟点、ago. 1990 , last month , half past six
I have been here since 1990. 1990起，我一直在这儿。
2.since+ 一段时间+ago
I have been here since five months ago。自从五个月前，我已经在这儿了。
3.since+从句
Changes have taken place since you left. 自从你走后，已经发生了很大的变化。
4.It is +一段时间+since从句
It is two years since I became an English teacher. 我成为英语老师有两年了。
since 和for区别
1.Since+时间点，具体时间
“自从、、、、以来，从、、、以后”用来说明动作起始时间
I have been in Beijing since 2010.
2.For + 时间段，用来说明动作延续时间长度，因此句中的谓语动词，也应该是延续性动词。
I have been in Beijing for one year。
We have learnt English for about three years.
I have been here for 3 days. (对)
I have arrived here for 3 days.（误）