-ING doubt: I have done is kiss(was kissing) my boss.

[Ray Child]

Hello. My name is Sabrina and I'm from Argentina.
I'm studying for an exam (again).
I took that exam and teacher made two corrections i don't understand. I wrote:

-"The silliest thing i have done is kiss my boss." The correction> "was kissing" my boss.
-"I think study geography is more interesting than study math". The correction is "studying" not study, the two times.

My question is> why use ING? Where can i find that rule? is this because there are different tenses in the sentence?

Thankyou very much.

[Mister Micawber]

For the first sentence, 'kiss' is fine, but your teacher thinks that the gerund 'kissing' is needed to match 'thing'. For the second sentence, the gerund (noun) is needed as the subject of the clause ('studying is interesting') and the second 'studying' is needed for parallel structure in the comparison: 'A is more interesting than B (is)'.
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Native English teacher at Mister Micawber's