I only Have a Question,the word "de"(also) is a word NOT a suffix,why do we use the major vowel harmony with it?,why does it become da?,I mean its
like the word Ev we dont make Av.
Please Help

Can you give an example in a sentence where it was change to ´da´?

" Ev " and "de" are different things.

 " Ev " is a noun [isim] whereas  "de" is :

1)  a suffix as locative case ---> Kafede  [at cafe],  Sarayda [ in [at] palace]

 So, it has to go with  the vowel harmony ---> Sarayda

2)  a conjunction  [also, as well ] ---> Ben de geliyorum.

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" Ev " is a noun and when it takes suffix, the suffix obeys the VH [vowel harmony] but  "Ev "doesn´t change.

Evde,  Evden, Evi, Evsiz, Evli , Evse

for 2) a conjunction [also, as well ] does de follow vowel harmony?

Are there other words that follow vowel harmony based on the preceeding word (besides mu)?

"de" can follow harmony only in two form -de or -da

Ben de  --> I also

O  da ---> He/she  also

Biz  de ---> We also

Onlar  da ---> They also

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*MI can take 4 forms according to vowel harmony:

-mi

-mı

-mu

-mü

*****************

Geldin mi ?

Kaldın mı ?

Oldu mu ?

Gördü mü ?

"de" alone is a word and yes it means also but when it´s added to a word like istanbul´da (in istanbul) or ev´de (at home) then it becomes a suffix and when it becomes a suffix then it means --> [in, on, at...]