It is +一段时间+since...意思是“自......发生以来已经一段时间了”。如：it is three years since he died.(自)他去世以来已经三年了。It is +一段时间+before...意思是“过了一段时间...才发生”如：It is 3 years before they accept the fact.过了三年他们才接受这个事实。
