People thought __necessary to invite Mr Smith to speak at the meeting
答案：it
翻译：人们认为邀请Mr Smith在会议上眼睛很有必要.
解析：it做形式宾语,真正的宾语是to invite Mr Smith
It was some time __ we realized the truth
答案：until
翻译：在我们认识到真相以前花费了一些时间
解析：根据句子意思使用until
Sandy could do nothing but__to his teacher that he was wrong(admit)
答案：admit
翻译：Sand除了向老师承认他的错误以外什么事情也做不了
解析：but当除了讲,后面使用to do,但是当but前面有do以及do的各种形式,后面省略to,直接加动词原形.