"He will not be here tomorrow."
Translation:Fydd e ddim yma yfory.
dydd = day, so your sentence would mean 'day he not here tomorrow'
If you meant 'Bydd e ddim yma yfory' then it is explained in the notes for the unit.
The negative form uses the soft version of 'Bydd' ie 'Fydd'
"ni fydd o yma fory" is also acceptable and would be used much more frequently in the North.
I was just about to say the same as Mabon. I'm from the north and "ni fydd..." is what we would say.