http://english.stackexchange.com/questions/106058/what-i-have-to-do-is-verb-inf http://english.stackexchange.com/questions/122277/omitting-the-last-to-in-all-i-need-to-have-to-must-do-is-to-do-somet https://en.wikipedia.org/wiki/Cleft_sentence#cite_note-1 Shirleen, 这几个网页的解释可能会有你需要的答案。

This is a tricky question. It may be to do with the construction of phrases required with cleft questions. If I discover the answer, I'll let you know. Perhaps others on her can explain - I'd be interested to know.

The grammar is fine here. :) All you need to do is think about it and use this chance to learn a new grammar rule. I wouldn't put a gerund here, as it wouldn't look like a gerund anymore.

Gary, I don't know how to explain this to a student. I may do some research but could you tell me what your thoughts were? Kind regards, Michael

@Gary I thought I could be a gerund?