it is 3 years since Tom has been abroad

it is 3 years since Tom has been abroad It is 3 years since he joined the army.为什么用is不用has been it is 3 years since he has been a doctor 怎么翻? It has been 3 years since he came here. it is two years since I came here还是it has been two years sin 看到一句It is 3 years since he joined the army.和一句It has been th Tom has been on the island since two years.(改错) It is the worst weather since 50 years ago为什么不是It has been.不 请问It is six years since I last met you 与 It has been six yea It's four years since Mike ____ to Tibet.A.has been B.is com It is five years since my father___?后面是用has been dead还是died? 分析一下它的含义?It is has been four years since he lived here．