543　It's/has been+一段时间+since从句

　这种句型意为“自从……起做某事已经一段时间了”。该句型中的since 从句通常用一般过去时。要注意的是，如果since 从句中的谓语动词是come, stop, leave, arrive等非延续性动词，句意为肯定，顺着翻译即可；如果是be, stay, meet, smoke, live等延续性动词，则要翻译成否定意思。

It is just a week since I arrived here. 我到这里才一个星期。

It is nearly twenty years since I lived in Yangzhou. 我离开扬州已经近20年了。（不在扬州 ）

It's been years since I enjoyed myself so much. 我已经有很多年没有那样痛快过了。

It is five months since I didn't smoke. 我戒烟已经5个月了。（不吸烟 ）

It is six years since he joined the army. 他参军已经6年了。

It has been ten months since she left home. 她离开家已经10个月了。