It is/has been…since… It was/will be…before… 这两个句It is/has been…since…It was/will be…before…这两个句型怎么用?要用什么时态?

It is/has been…since… It was/will be…before… 这两个句It is/has been…since…It was/will be…before…这两个句型怎么用?要用什么时态?
It is/has been…since… It was/will be…before… 这两个句
It is/has been…since…
It was/will be…before…
这两个句型怎么用?要用什么时态?

It is/has been…since… It was/will be…before… 这两个句It is/has been…since…It was/will be…before…这两个句型怎么用?要用什么时态?
（1）It is +时间段+since 从句 （是规范说法）
（2）It has been +时间段+since 从句 （口语说法）
(3) 他们表达意思都是相同,都表示：自从.起,自从.以来,已经有多长时间了
（4）若since引导的状语从句的谓语动词是终止性的过去时,则从句表示的时间是“从动作开始的那一时刻起”.
如：He has studied very hard since he came to our school.
自从他来到我们学校,他学习就非常努力.
We have been missing them since they left here
自从他们离开这里,我们就一直很想念他们.
（5）若since引导的状语从句的谓语动词是持续性动词或表示状态的动词的过去时时,则从句表示的时间是“从那持续动作或状态结束时算起”.如：
I haven’t heard any noise since I slept.Sleep 为持续性动词,sleep的动作结束时,即“醒来”时,这句应译为“我醒后还未听到任何声音”.
John is now with his parents in New York,it is already three years since he was a teacher约翰现在和父母一起住在纽约,他不当教师已经三年了.
How long is it since you lived in Shanghai?你离开（没住在）上海多久了?
It is half a month since he was a League member.他退团（不当团员）半个月了.
It’s been quite some time since I was last in London我离开伦敦已很长时间了.比较：He has written to me frequently since I was ill.自从我病愈以来,他屡次给我写信.（从句谓语为状态动词）