"I'm writing to him today."
Translation:Dw i'n ysgrifennu ato fo heddiw.
ysgrifennu at = "to write to"
But the word at changes before fo "him" to become ato:
ysgrifennu ato fo = "to write to him"
You use specific prepositions with specific verbnouns - you learn them together as a pair. So in the case of ysgrifennu, you learn ysgrifennu at for "write to (a person)". Or in the case of dweud, you learn dweud wrth "say to / tell". It's a much better way than trying to remember each element separately.
No. You ysgrifennu at a person and ysgrifennu i a thing or place:
Dw i'n ysgrifennu at Siôn "I'm writing to Siôn"
Dw i'n ysgrifennu ato fe "I'm writing to him"
Dw i'n ysgrifennu i'r cwmni "I'm writing to the company"
Dw i'n ysgrifennu i'r papur bro "I'm writing to the local paper"
You get the same pairs with other verbnouns:
mynd/dod/anfon at "go/come/send to (someone)"
mynd/dod/anfon i "go/come/send to (somewhere)"
sgwennu is a spoken dialect form of ysgrifennu. Duo should not really be putting forward as a correction to a mistaken answer - I’ll see if I can stop that happening again.
ato fe is correct. I cannot check the database entry at the moment as the system seems to be running very slowly. There may be some background maintenance going on - we are expecting Duo to be doing some work on the Welsh course this week - and perhaps that is also causing other issues.