"The sheep is eating grass."
Translation:Tha a' chaora ag ithe feur.
It's a feminine noun with the definite article. This is actually literally doing my head in and I have printouts of two separate crib sheets purporting to explain it but I'm sure I'll never remember it.
But the explanation for this is in the Tips and Notes for the "Animals" skill. A feminine noun beginning with a C is in "category 1" which form the definite article by taking a' and lenition. There are four categories. That is in addition to the three categories of masculine nouns that do different things to form the definite article. Even the categories are different. I hope you have better luck mastering this than I'm having.
You might also want to join the Facebook group 'Scottish Gaelic Duolingo'. Often helpful answers from native speakers. Sometimes quite funny. We joke about Iain who steals underpants! It's here. https://www.facebook.com/groups/ScottishGaelicDuo/?ref=share