1.It__as if they could see and hear us in action?为什么选will be

1.It__as if they could see and hear us in action?为什么选will be不选was
2.Until recently historians assumed that calendars__with the advent of agriculture为什么选come to existence是因为existence是名词吗?
3.a student had dressed up as a policeman and was amusing himself__
为什么选at other people's expense不选by making fun of people
4.不知道有谁记得,新概念3里面有个地方用as if后面加动词原形,请问这是怎么回事?有这种用法吗?
5.请问如果物主代词比如his在介词后面要不要写成him，（这样不是不能代表“他的”的意思啊?）还是原来的his？

1.这个大概考察as if 引导表语从句吧.
表语从句位于连系动词后,有时用as if引导.其基本结构为：主语 + 系动词 + that从句.

2.come to 中的 to 是介词,后面需要宾语.existence是名词,可做它的宾语.
3.最后那半句大概是说牺牲别人来取悦自己.

4.不记得.
5.依情况而定,不一定要改写成him.
这个问题你可以从字典上找些例句看看,也许有帮助.