My students crack me up

Yesterday I spent some time explaining to the students how to identify an infinitive. "Beware!" I told them. "They don't always come with a to in front of them. To go, to eat, to play: all of those are easy to spot. But sometimes the to is omitted. And then you have to look for other clues to see if your verb is an infinitive or a finite form."

I explained what these other clues were, then gave them a quick test. "In the sentence I must go now, is go a finite form or an infinitive?"

"An infinitive!" One of the students answered immediately.

"Excellent," I praised her. "And how do you know?"

She beamed proudly. "Because there is an omitted to."