"My grandpa is a police officer."
Translation:He mākaʻi koʻu tūtū kāne.
I'm just a beginning student too, but the impression I get is that a sentence can begin with "he", but maybe is not allowed to start with the noun articles "ka" or "ke", so when you would otherwise start the sentence with "ka" or "ke", you have to add ‘O in front of it. If I'm not mistaken there are verb particles "ka" and/or "ke", so perhaps this prevents confusion between the verb particles (which would be able to start a sentence) and the noun articles (which must be preceded by ‘O at the start of a sentence). I could be wrong, but that's the impression I get.