In Turkish, suffixes are formed according to Turkish vowel harmony, rules whereby most vowel sounds in a word are made either in the front of the mouth or the back, but not both.
if the vowels in the root are formed in the back of the mouth (a, ı, o, u), as in banka (bank), you add -lar to make bankalar (banks).
If the vowels are made in the front of the mouth (e, i, ö, ü), you add -ler to tren to make trenler (trains). Likewise, arabamız, ‘our car,’ but otobüsümüz, ‘our bus’.
a ı o u —> a ı o u
e i ö ü —> e i ö ü
The suffix is the same one, but modified according to vowel harmony. It’s not nearly so weird as it seems once you get the hang of it, and it gives a nice mellifluous sound to Turkish sentences.
Don’t worry about the vowel harmony after a time it is about make sense that you do the harmony without giving attention.