"I write letters in the morning."
Translation:Jag skriver brev på morgonen.
The translation said 'i morgon' was also right, so now i'm really confused...
in the morning is ambiguous in English, it can mean both 'tomorrow' and literally 'in the morning'.
When to use i or på with time is a bit tricky. Swedish simply wants på with morgonen. Very roughly, you could say i is used for the past, and på with the future or in general.
That would be 'See you tomorrow' and 'See you in the morning' respecively. If we would want to say 'See you tomorrow in the morning' we could say 'Vi ses i morgon på morgonen' but that sounds a bit clumsy. Instead we say 'Vi ses i morgon bitti'. Bitti is an adverb meaning early.