Rex soon became an expert at opening the gate,at后可以用不定式吗?为什么

become an expert at 与become expert at 与 Someone is waiting foryou at the gate 为什么用is BARBARIANS AT THE GATE怎么样 at the school gate 同义词 STRANGERS AT THE GATE怎么样 Do you know the name of the boy standing at the gate?为什么用sta 是be an expert in/at/on ,还是be expert in/at/on? at gate和at the gate有什么区别? Mr Black（ ）(arrive)soon.Let us wait for him at the gate an expert in/on/at有什么区别 an expert in/at/on/with有什么区别? 为什么often wait for me at the school gate often这个频率副词后加了wait的动