ad. （常用於現在完成式或過去完成式）

1. 此後；從那時到現在
   She left school three years ago and has worked as a nurse ever since.

   她三年前畢業，從那時起便一直當護士。

2. 之前，以前

prep.

1. （常用於現在完成式或過去完成式）自……以來，從……至今
   I haven't heard from him since last year.

   我自去年以來未曾收到過他的信。

conj.

1. （常用於現在完成式或過去完成式）自……以來，從……至今
   It's been ten years since they married.

   他們結婚到現在已經十年了。

   She has been living a hard life since her husband died.

   自從丈夫死後，她一直過著艱苦的生活。

2. 既然；因為，由於
   He must have taken the book since it isn't here.

   他一定是把書拿走了，因為書已不在這裡了。

   Since it is so hot, let's go swimming.

   既然天氣這麼熱，我們去游泳吧。

prep.

conj.

1. during or in the time after.
2. for the reason that; because.

adv.

1. from the time mentioned until the present or the time under consideration.
2. ago.