ad.(常用于现在完成时或过去完成时)

1. 此后；从那时到现在
   She left school three years ago and has worked as a nurse ever since.

   她三年前毕业，从那时起便一直当护士。

2. 之前，以前

prep.

1. （常用于现在完成时或过去完成时）自……以来，从……至今
   I haven't heard from him since last year.

   我自去年以来未曾收到过他的信。

conj.

1. （常用于现在完成时或过去完成时）自……以来，从……至今
   It's been ten years since they married.

   他们结婚到现在已经十年了。

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

   她自从丈夫死后，一直过着艰苦的生活。

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

   他一定是把书拿走了，因为书已不在这里了。

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

   既然天气这么热，我们去游泳吧。

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.