I feel like Duo is trying to teach us something with these type of sentences but I'm not so sure what... Like in the one 'the garage is a cold garage'
It's trying to teach the basic form of one of the two basic sentence types in Arabic, the nominal sentence.
Verbal sentences, any sentence with a verb, can be just a verb, or have nouns also, and they can come in any order, but traditionally VSO was considered standard. SVO is very common today.
But any Arabic sentence that lacks a true verb, like this one, which simply combines nouns and adjectives is called a nominal sentence, with the basic form "subject + description". The subject and the description can each either be a noun by itself, an adjective by itself (which can imply a noun), or a combination of nouns and adjectives.
It's also trying to show us case agreement between nouns (and adjectives, which work basically the same way). In standard Arabic (but NOT spoken, vernacular Arabic) nouns and any other adjectives or nouns describing them must all have the same final vowel: "u" if it's the subject, "a" if it's the gramattical object, and "i" if it's in the genetive case. (For more on the genetive case, which shows posession and relation between nouns, google "iDafa")
Nouns and their adjectives must also agree in definiteness, so if "al", "the", comes before the noun, it should also come before the adjective, or alternatively, if it's indefinite, the noun and it's adjective(s) should both end in "n", after the final vowel (only in standard Arabic).
BUT as you can see by this sentence, even though a subject in a nominal sentence must have the same final vowel as the noun(s) (/ adjective(s)) describing it, it does NOT have to agree in definiteness, (or number or gender). AND the final vowel mentioned before gets dropped at the end of a phrase. That last part is pretty crucial for showing where the end of the sentence is in standard Arabic.
In dialects, the Ns and vowels at the end of the word aren't normally pronounced, but Duo hasn't really figured out how to teach those differences yet.