问大家两个关于since和for的选择题My uncle has been living here for twenty years.用since把后三个单词改写成4个单词Marcia skated_____in last year's marathonA.for an hour B since one hour ago应该选什么?为什么选它?为什么不选另一个?

A 因为since 通常接从句，接短语时，意思是自从...以来，意思和这里的句子不符合

My uncle has been living here since twenty years ago .
choose A

My uncle has been living here for twenty years. 用since把后三个单词改写成4个单词
答：My uncle has been living here since twenty years ago.
My uncle has been living here since the year 1991.
Marcia skated_____in last year's marathon
A.for an hour B since one hour ago
应该选什么？为什么选它？为什么不选另一个？
答：A
析：因为since用现在完成时。for+一段时间 还可以用于过去时态。

since twenty years ago
选a 在去年的马拉松比赛中，marcia 滑了1个小时
b无法翻译

My uncle has been living here since twenty years ago.
Marcia skated_____in last year's marathon
A. 玛西亚在去年的马拉松上滑了一个小时的冰.
不应选B,因为b意思是“自从一个小时前开始,一直持续到现在”,和句子的过去式及过去时间状语不一致.

since twenty years ago
选A 在去年的马拉松比赛中，Marcia 滑了1个小时。
for可以用于一般过去时，since表示时间的时候只能与完成体连用。