有since 或者是for 一定是过去完成时么

问题描述:

有since 或者是for 一定是过去完成时么

不一定了
I have studied here since l left home
I studied here for three years怎么区分 我过去式和过去完成时 总是分不清...那就要看具体的题目而定啦555555......虽然不太明白 不过还是谢啦~