since

1、since then意为从那时以来，通常与动词的现在完成时连用，指从过去某一时刻起到现在为止的一段时间。2、Since then 前后一般接现在完成时，从那以后，现在完成时表示过去的某件事对现在还有影响。

Since then, he has developed another bad habit.从那以后,他养成了另一种坏习惯。He hasn't come close to those numbers since then.自那以后，他从没有达到那赛季的表现。Exactly three years have passed since then.从那时起，整整3年过去了。