■ First category is thick voiced (back vowels):

□ And second category is subtle voiced (front vowels):

" e, i, ö, ü "

<em> </em>So if we want to make a word plural we should add

-lar or -ler suffixes, according to word's last vowel.

For example:

"Kahve/Coffee" word and we should pay attention to last vowel "e" here (kahve / it is the second category), so we will add to -ler suffix here. Then plural form of "kahve" will be "kahveler"

Another example:

"Çocuk/Child" word we should pay attention to last vowel "u" here (çocuk / it is the first category), so we will add to -lar suffix here. Then plural form of "çocuk" will be "çocuklar"