"Her son's book."
Translation:Leabhar a mic.
Very new to Gaeilge here! Point of confusion is in the background notes on Duolingo for this module. "The genitive singular form of FEAR (FIR) is the same as the plural (FIR)"
So if I apply this to MAC, the genitive singular form should be the lenited form MHIC, compare FHIR in the table.
So why isn't the answer "Leabhar a mhic"?
There are a few things going on. In the first place, you can't always apply the singular/plural rule that way to derive the genitive, but because fear and mac are 1st declension nouns, it works in this case. It doesn't work for bean, buachaill, cailín or iníon, for example.
Secondly, the singular genitive of fear is fir. The notes only show it lenited after an (in contrast to the nominative case, where feminine nouns are lenited after an). So hata fir - "a man's hat", hata an fhir - "the man's hat".
There is no an here to cause lenition, so the lenition in leabhar a mhic would have to come from the possessive adjective a, which would make it "his" rather than "her".