until
till和until一般情况下两者可以互换,但是在强调句型中多用until.并且要注意的是：如果主句中的谓语动词是瞬时动词时,必须用否定形式；如果主句中的谓语动词是延续性动词时,用肯定或否定形式都可以,但表达的意思不同.例如：
I didn't go to bed until（till） my father came back.直到我父亲回来我才上床睡觉.
It was not until the meeting was over that he began to teach me English.直到散会之后他才开始教我英语.
I worked until he came back.我工作到他回来为止.
I didn't work until he came back.他回来我这才开始工作.
Please wait until I arrived.在我到达之前请等我.
since
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.
