"The fridge is not cold."
Translation:Chan eil am frids fuar.
The form of the article depends on a few thing – the grammatical gender of the noun (masculine vs feminine), its number (whether it’s singular or plural), its case (nominative, dative, genitive), and the first sound of the noun.
Generally when the article would have the form an it changes to am before the labial consonants: B, F, M, P.
And it changes to a’ if it is followed by a lenited consonant (but not by a vowel).
It is explained in the tips and notes to the lessons introducing various forms of the article. You can find the tips and notes in the web browser version of Duolingo – on https://duolingo.com (unfortunately not in the mobile app). You can also read them on https://duome.eu/tips/en/gd.