"His suit comes in a month."
Translation:Hans kostym kommer om en månad.
How do you know when to use på or om? If I remember correctly, both can mean "in."
om with time means 'after some time period', like in this example. It's English that uses in for both 'after a certain amount of time' and 'during', as in in November.
Is there a good resource i could use to learn and read up on all the different om/pa/i uses. The lessons here are apparently not quite doing it for me.
No, sin always refers back to the subject, but here His suit is already the subject, so that is not possible here. You could say Han får sin kostym om en månad ('He gets his suit in a month') because then Han would be present in the sentence.