Learn Turkish: Onlar yeni araba satıyor. - They are selling new cars.

The root of the verb 'to sell' is sat-. Because it ends in a consonant, we have to insert a vowel before the -yor suffix. According to 4-way vowel harmony, the 'a' in sat- means we use -ı-, giving us satıyor.

Since the subject is Onlar (they), why doesn't the verb end in -lar (satıyorlar)?

In Turkish, if you already state the plural subject Onlar (they), you don't actually have to make the verb plural too! Onlar ... satıyor is perfectly correct and very common in everyday speech. You could also say Onlar ... satıyorlar, but it is completely optional.

The translation says 'cars', but araba doesn't have the plural suffix -lar. Why is it singular?

When talking about a direct object in a general, unspecific sense—like 'selling cars' as a broad activity—Turkish keeps the noun singular. You would usually only use the plural arabalar if you were talking about a specific, known group of cars.

If they were selling specific cars and we used the plural arabalar, would the adjective yeni (new) change?