since的问题—Doyousmoke?—No，Idon't.ButIusedtosince的问题—Doyousmoke?—No，Idon't.ButIusedto.It's20yearssinceI().()内应该填写什么时态？完成时吗？？还是用didn'tsmoke/havesmoked/begantos

Question

since的问题 —Do you smoke? —No，I don't.But I used to
since的问题
—Do you smoke?
—No，I don't.But I used to.It's 20 years since I ( ).
()内应该填写什么时态？完成时吗？？还是
用didn't smoke/have smoked/began to smoke/smoke
从语意来讲我觉得是didn't smoke但since作自从好像接完成时？？？
还有个问题As a student studying in America，I find that the method of teaching is quite different from () usedby Chinese teachers.
()内是that还是what？？为什么

Answer 1

同指教学方法。因为很多时候that和what能通用