it's +时间+since/that/when/before句型用法的差别

1. It's + some time + since从句.
翻译：“自从...以来已经多久了”.
要求：since从句的时态必须是一般过去时.
如：It's ten years since I graduated.
自从我毕业以来,已经十年了.
2. It's + some time + that从句.
翻译：“...多久了”.
要求：此为强调句型,去掉It's和that之后,剩下部分必须是个完整的句子.
如：It's ten years that I've lived here for.
还原成：I've lived here for ten years.
我在这儿居住已经长达十年了.
3. It's + sometime + when从句.
翻译：“...是在某个时刻”.
要求：when从句是定语从句,要求从句的时态和It's的is/was对应.
如：It was ten years ago when I graduated.
我毕业是在十年前.
4. It's + some time + before从句.
翻译：“过了多久...才...”.
要求：before从句的时态必须是一般过去时；It's = It was.
如：It's ten years before I graduated.
过了十年后,我才毕业.