这些主要是用来引导时间状语从句的连词。一般地说,主句如果是一般现在时,从句可根据需要用任何一种时态。若主句用了一般过去时,从句就只能用过去的某一种时态。但是since从句中就往往是过去时态。如:He has been in NewYork since he graduated from University.
while when 都是当...时的意思 while一般用于并列进行时 如:I was doing my homework whili you calling me as soon as 意思是 一...就 注意"主将从现" 如:I will call you as soon as I go to beijing