1.  It + be + 时间 + since-clause

这个句型表示从since 从句谓语动作发生以后到现在或过去所经过的一段时间，意为“自从......以来已多久了”，主句多用一般现在时，从句用一般过去时，如果表示过去的情况，主句用一般过去时，从句用过去完成时，或主句用过去完成时，从句用一般过去时。例如：

It is three years since his father passed away . (从现在算起)

It was 10 years since they married . (从过去算起)

It had been fifteen years since she left Shanghai . (从过去算起)

30 years has passed since we joined the Party . (从现在算起)