It+be
+时间+before/since/when/that
句型辨析
I.since
1.It is/has been+
时间段+since
sbdidsth某个动作发生持续多久
eg:Itis/has been three years since he finished the work.
自从他完成这项工作已经三年了
.2.该句型中若
since
后跟延续性动词，要翻译成否定含义
,即“没做某事已经多久了
”.eg:It
has been three years since he worked here.
他不在这工作已经三年了
.It has been three years since he smoked.
他已经戒烟三年了
.但是：It has been three years since he began to smoke.
他吸烟有三年了。
II.before
.1.It was/will be+
时间段+before...
过多长时间将发生某动作或事情；或过了多久发生了某事或动作。
eg:It
was three years before he finished ...