Ben kötü-y-üm
how did they use the fusion consonant & we learn that if there are a vowel from the same group we droppe one of them:

If both of the vowels are in the group "-i, -ı, -u, -ü" than these two vowels have to be the same (look at the vowel harmony rules to understand why). Since the two vowels are the same, it does not matter which one we drop in this case.

i think it´s not false if i say: ben kötüm

 it is completely not correct to say :ben kötüm

and we do need here to say

"ben kötüyüm "

b.rgds/ally

 It is definitely wrong!

You are lucky there isnt a word "köt", otherwise you would have said you are one of them.

mmmmmm thx

Suffixes are defined by putting the optional consonants (or vowels) between paranthesis.

For example -(i)m (i represent ı,i,u,ü one of which you would pick according to vowel harmony rules) means after a word ending with a consonant you would add one of ı/i/u/ü and if the word ends with a vowel you would only add m

anne=mother

anne & -(i)m -> anne-m (anne ends with  vowel so use -m)

müdür=manager

müdür & -(i)m -> müdür-üm (müdür ends with a consonant so add ü before m)

On the other hand -(y)im means after a word ending with a vowel you would add y and if the word ends with a consonant you would only add im

anne=mother

anne & -(y)im -> anne-yim (anne ends with  vowel so use -yim)

müdür=manager

müdür & -(y)im -> müdür-üm (müdür ends with a consonant so add -üm)

-(i)m means my

-(y)im means I am

So

anne-m = my mother

müdür-üm = my manager

anne-yim = I am (a) mother

müdür-üm = I am (a) manager

Notice second ones are the same but have 2 different meanings depending on the suffix.