"You are not buying trousers."
Translation:Chan eil thu a' ceannach briogais.
Since there doesn't seem to be a good explanation.. Thu doesn't turn into ort at all, it's just never there for the earlier sentences. It's always tha còta and orm is a verb referring to the subject later in tge sentence. For 'Tha còta orm', orm is what the còta (the subject, coat) is doing (being worn by the speaker). If you said tha mi orm you're saying you're wearing yourself which is... technically the truth?
Ort/orm means a person is wearing the subject and the word itself tells you who is doing the wearing. Tha còta orm = a coat is being worn by me = I'm wearing the coat (simplified)
Hopefully that makes sense and I'm not completely wrong about lyfe, which is statistically likely