My sister has been a teacher for two years.
或者 My sister became a teacher tow years ago.
for + 一段时间 通常是完成时态的标志 become成为 作为一个瞬时性动词不能在完成时态中与一段时间连用 所以用been替换
My sister has been a teacher for two years.
或者 My sister became a teacher tow years ago.
for + 一段时间 通常是完成时态的标志 become成为 作为一个瞬时性动词不能在完成时态中与一段时间连用 所以用been替换