（1）since用于完成时态，既能用作介词，也能用作连词，后常接时间点，意思是“自从”。例如：
He has been a worker since he came into this city.自从他来到这个城市，他就是工人了。
I have never seen him since we last met in Shanghai .自从我们上次在上海见过之后我再也没见过他。
since作连词，还有“既然”的意思。例如： Since you are interested in it, just do it. 既然你对它感兴趣，那就做吧。 You can have fun now since you’ve finished your work.既然你已经做完了功课，就开心玩会儿吧。
（2）for用于完成时，用作介词，后常接一段时间，意思是“经过…”。例如：I have learned English for five years.我已经学了五年英语了。They have waited for you for 30 minutes.他们已经等了你三十分钟了。
for也可以用作连词，但意思是“因为”。例如：They missed the flight for they were late.他们由于完到了而误了航班。He fell ill for many reasons.他由于多种原因病倒了。