http://merhabaturkish.com/learn/grammar/when-to-add-dir-dir-dur-or-dur/

However, if the word ends in one of those consonants [f, s, t, k, ç, ş, h, p] , the suffixes are tır, tir, tur, tür

[f, s, t, k, ç, ş, h, p]

fıstıkçı şahap

Thank you for adding this, Haydi. One of the mysterious things in Turkish syntax. But usually you can´t make cardinal mistakes with -dir and that´s why we don´t feel desperate need to clarify it.

Interestingly, the article stresses that -dir expresses certainty which roots from the facts. But when you feel you have to stress the certainty there is already a doubt, isn´t there? gokuyum has said something about this somewhere...one day when I tried to sprinkle -dir´s in every sentence...here it is:

Vikipedi explains -dir this way: creates the aorist of every tense. Not very clear to me. What on earth could yazıyoruzdur mean?

Geniş Zaman [değiştir]

Türkçede herhangi bir zamanın genişini (-dir) eki yapar.
 

Türkçede Geniş Zaman Tablosu (yaz-):