The words since and for both deal with time. The two words are related to a duration or time period. Both words are prepositions (although since is also an adverb and a conjunction).SInce和for的使用都与时间有关。这两个词都与时间段有关。两个词都是介词(但since也是副词和连词)。For example,例如,I have been teaching English for 10 years.我教英语已经10年了。orI have been teaching English since 2007.我从2007起开始教英语。FOR:The word for also sounds like the number 4. This reminds me of mathematics.For听起来也像是数字4的发音。这让我想起了数学。When you do the math (or calculate the amount of time) for the person then you use FOR.当你为某人计算数学题目(或计算时间)时,则使用FOR。For example:例如:I have been waiting for 10 minutes.我已经等了10分钟了。She has lived in the United States for 20 years.她在美国住了20年。He has been raising his hand for an hour.他已经举起了一个小时的手。SINCE:Since generally refers to the beginning of some period.Since一般指的是指某一时期的开始Let's look at some examples:让我们看一些例子:She has been working out since 2:15pm.她从下午2:15开始一直在工作。They have been studying medicine since 2016.他们从2016年以来就一直在学医。I have been living here since 2013.我从2013年以来一直住在这里。I haven't seen Sarah since Friday.自从星期五以来我就没见过莎拉。One last thing about since.关于since的最后一点。If you use the sentence, "It has been… (years or some time) then you MUST use since when using Present Perfect Verb Tense.如果使用这个句子:"It has been…(某年或某个时间)时,因为该句为现在完成时态时,所以必须使用since。For example,例如:It has been 5 years since I quit smoking.我戒烟已经5年了。Next month, it will have been 4 years since I have been living in Brazil.到下个月,我已经住在巴西4年了。