Question

for+一段时间 究竟什么时候用一般过去时，什么时候用完成时态呢？ 它跟since的区别？

Answer 1

since+时间点，一般用在完成时态句子中。for+时间段，用在完成时态或者过去时态句子中。eg:我学英语两年了I have studied English for 2 years.I have studied English since 2 years ago.自从两年之前我就开始学英语了。I watched TV for two hours last night.我昨晚看了两个小时的电视。

Answer 2

since后面是时间点
for后面是时间段
比如：我学英语两年了I have studied English for 2 years.两年，时间段=I have studied English since 2 years ago.两年前时间点

追问

为什么for+一段时间的时候它也会用一般过去时呢？