"I am not buying pants."
Translation:Chan eil mi a' ceannach briogais.
I think most do require the 'mi' in the examples you give assuming the speaker is doing the thing. 'Mi' means I or me. But 'I have a coat on' is 'Tha còta orm'. 'Orm' means 'on me' so the me/I bit of the sentence is hiding in the preposition. 'Ort' is 'on you,' and there are different words for on her, on him, on them etc ts a bit more complicated than this as the grammar is very different from English but that's why there's no 'mi'. 'Leam' is with me, 'leat', with you and so on, so again the me bit is hiding.