"I do not have a shirt on."
Translation:Chan eil lèine orm.
I'm just guessing so someone can correct me if I'm wrong. But so far it seems like the ones that don't need the "mi" are transitive verbs. That is the verb is split up: Tha _ orm Chan eil _ ort
So those verbs include the "mi" or "thu" already. In fact, it seems likely that's what the m and t at the end actually stand for.