"Will you be at home tomorrow?"
Translation:Fyddwch chi gartre yfory?
Strictly speaking, "adre" means "homewards" rather than "at home". However, I hear it used that way in speech all the time.
The Tips & Notes say that 'n/yn usually follows the pronoun here so I put Fyddwch chi'n gartre yfory but chi'n was marked incorrect. Why am I wrong?
'n/yn doesn't belong before "gartre".
'n/yn comes before a noun, verb or adjective. "gartre" is an adverb in this sentence.