-e type suffixes are the suffixes that have the -e and -a versions.  According major vowel harmony, back vowels are followed by back vowels and front vowels by front vowels. Therefore vowel harmony in -e type suffixes works like this:

If the last vowel is "a, ı, o, u" the suffix will be the -a version

If the last vowel is "e, i, ö, ü" the suffix will be the -e version

Examples to -e type suffixes:

-ler, lar (plural suffix)

-mek, -mak (infinitive suffix)

-de, -da (locative case suffix)

Looking at the last vowel of each noun, choose the correct plural suffix. The plural suffix is either -ler or -lar according vowel harmony.

Example:

kitap : kitaplar