Vowel harmony is the reason Turkish suffixes seem to "change." They don't change randomly — they adapt to the last vowel of the word. Learn the pattern and it becomes effortless.
Two-way harmony (e / a)
Many suffixes have an "e" version and an "a" version. Front vowels (e, i, ö, ü) take e; back vowels (a, ı, o, u) take a. Example: ev-ler (houses) vs kitap-lar (books).
Four-way harmony (i / ı / u / ü)
Other suffixes have four versions. The vowel matches both the frontness and the roundedness of the previous vowel. It's a small table you internalize with practice.
Why it's worth learning first
Once harmony is automatic, plurals, cases, tenses and possessives all follow the same logic. A tutor can drill this with you in a couple of focused sessions and save you months of confusion.