I am confused by the specific function of the -tur and -dur suffixes.  

For example kötü becomes kötüdur?

I can be wrong but for me it´s :

O kötü = He is nasty

O kötüdür = He "must" be nasty

Yunus evde = Yunus is at home

Yunus evdedir = Yunus "must" be at home

But wait, because I´m not a native speacker

This is - dir suffix. And it can take many forms like dır/dir/dur/dür/tir/tır/tur/tür

It has the function of am/is/are in Turkish. But we dont always use it.

"O güzel" and "O güzeldir" means the same thing. We also use this suffix to express a guess. Then "O güzleldir" gains a new meaning. It is "I guess she is beautiful" So you can ask me how you will understand if it is a guess or not. You can understand it reading the whole dialogue or easy way is because we dont use it like am/is/are most of the time, there is a big chance of it being a guess.

This is called "önerme" in Turkish.

For example

Buz soğuktur (ice is cold) is a fact.

Dünya küre şeklindedir ( earth is a spehre) is a fact.

two plus five is equal to seven = iki artı beş yediye eşittir

o evdedir is like a guess. someone asks you "Ahmet nerede?" and you do not know the exact answer and would say "evdedir". which means Ahmet is probably at home but he maybe not at home, who knows? you just guess.

Something like that...

Thank you so much! That helps a lot!

-dir 

dır -dir -dur -dür 

tır - tir - tur - tür

When -dir suffix attached on predicates [noun/verb predicate ] , it adds"possibility/guess " or "certanity" meanings.

- Şimdiye eve varmıştır. ---> He/She probably arrived home by now.[ probability ]

- Sigara sağlığa zararlıdır. --> [Smoking] Cigarette is bad for health.

[ It is a certain fact that smoking is bad for health ]

- Babam çok iyi bir insandır. ---> My father is a very good person. 

[ This is a certain fact that I belive, that my father is a very good person]

- Dedemi uzun zamandır görmedim. Yaşlanmıştır. ---> I haven´t seen my grandfather, He probably got older. 

[ It is strongly possible that my grandfather got older]