since后面能跟瞬间动词的完成时态吗？

譬如 it has been 10 years since they left.

It is 3 years since they have seen/saw each other.

有关since从句中的瞬间动词与持续动词，我补充两点：

1.since从句中若是用的非持续性动词，则无论在since前加ever还是将since从句中的谓语改为完成时态来加大语意，从句中谓语动词的动作都没什么持续性。比如：

My granny has been catnapping ever since the film began / since the film has begun. 自电影刚开始，我奶奶就一直在打瞌睡。

2. 在“It is + 时间 + since-从句” 的结构中，状况则不同。它表示从过去某一时刻起到目前（说话时刻）为止的一段持续时间的总和，从句中一般都用一般过去时，但无论用持续性还是非持续性动词都表示动作或状况的完成，时间的起点应该是在动作或状况完成的那一刻开始算起。比如：

① It is six years since I joined the Party. 我入党已经有六年了。

② It is six years since I was in Beijing. 我不在（离开）北京已经有六年了。

在句②中，无论在since从句前加ever还是在从句中用have been都没办法表达“我已经在北京呆了六年”的意思。又如:

It is about five years since my father smoked. 我爸爸不抽烟已经约五年了。