It's 20 years since.It's 是is 还是has

it's years since i It's 20 years since.It's 是is 还是has it's seems like years since it's been It's three years since he was married .的翻译 It's years since she drove anywhere for pleasure 翻译 求解英语翻译：It's 2 years since he smoked. It's years since I enjoyed myself so much 翻译 It's years (since) I enjoyed myself so much.Why? it's five years since i______ (join) the League 翻译 It's five years since he smoked It has been 2 years since we worked here It's two years since he left his hometown.since 为什么不能换成after It's years since I enjoyed myself so much.其中since是什么用法? I haven't met him since 1999.=It's ( ) years since I ( ) him. 英语翻译还有这句 It'It's three years since I was a solder.It'It's three years since I joined the army It's years since l enjoy myself so much.since可否换成before?就像it's not long before since引导的时间状语从句为什么出现在一般现在时里而不是现在完成时?我在这个句子里发现的：It's 20 years since he joined the party.既然有since了,前边为什么用It's呢? It's 3 years since I saw him last time.能不能换成it was... 书上说since不与瞬间动词连用,那it's 2 years since he left his hometown为什么对?