.It+be+ 时间+before/since/when/that句型辨析I .since1. It is/has been+时间段+since sb did sth某个动作发生持续多久eg:It is/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
