This may sound silly to many of you but ive been trying to figure this out from quite some time and İ stıill dont know the answer. why in geniş zaman in words such as öpmek or gülmek or etmek we write say öpErim or gülErim or edErim. Of course İ know it has something to do with the vowel harmony but the question is, when should İ stick to minor and when to the major vowel harmony ?

Will be grateful for some prompt response as İ cant wait to find out the answer

"geniş zaman" suffix is -(e)r/-(i)r. It has some irregularities (not so scary) and you need to memorise some verbs.

Here´s some summary:

if a verb ends with a vowel

This is very easy and regular. Just add -r suffix:

ara-r, dile-r, başla-r, yara-r, tanı-r etc.

if a verb ends with a consonant

Monosyllable verbs usually take -er with a few -ir ones.

at-ar, bat-ar, bit-er, kap-ar sat-ar, sil-er, yet-er ...

but

al-ır kal-ır gel-ir bul-ur bil-ir ol-ur öl-ür ver-ir var-ır vur-ur dur-ur gör-ür (memorise these 12)

and a derived verb takes -ir even if monosyllable (they are not many - just 2 or 3):

ye-n-ir (compare it to yen-er), de-n-ir, ko-n-ur

And polysyllable verbs takes -ir, and there is no exception of it.

tanıt-ır, getir-ir, bırak-ır, salın-ır, yetiş-ir ...

derdiniz ne sizin ?