since 后加时刻,两年前是一个时刻,for后加时间段,所以不能加 two years ago ,只能说 for two years
two years ago 两年前 是个时间点, 而不是【时间段】, 所以用since,即从2年前的那个时候算起
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since two years ago等于for two years
eg:she has been a teacher since two years ago.她在两年前就已经成为一名教师了。
she has been a teacher for two years.她成为一名教师已经两年了。
点时间 和段时间 的区别