It's+been++a+long+time.been在这里起什么作用?为什么要用been

It's+been++a+long+time.been在这里起什么作用?为什么要用been It's been a long Hi Mary It's been a long time .语法填空 找一首歌.歌词里有一句“it s been a too long time” 一个英文句子解释it's really been a while,try again.这里为什么要用been.为什么不用 what a long time he's been waiting,isn't it?对吗,为什么 It's a long time since还是It has been a long time since It`s been such a long week. it's been a long day句型 关于完成式和过去式It's been a long time since I've seen it.It's been she had been at the doctor's for a long time为什么用doctor's we have been best friends for a long time中friends为什么要加s