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.
since是在...之后,从..以后/以来 before 是在...之前 你说的那两个词都是在句中做连词 It is a long time before the building was accomplished. 这个建筑花了很长时间才建立起来 It has been a long time since Susan said that she want ...