"The girls drink water."
Translation:Kızlar su içer.
My understanding is that most Turkish words will contain "front" vowels (i, e, ü, ö) or "back" vowels (a, ı, o, u), but not both, and suffixes change to fit this pattern. In this case, if the last vowel in the nominative singular is a front vowel, the suffix is -ler. If it is a back vowel, use -lar (Notice that the E in -ler is also a front vowel and the A in -lar is also a back vowel).