In many resources, "-dir" is shown as the personal suffix of 3rd singular. But, indeed, this is not correct. The 3rd singular person has no personal suffix.

One of the tasks of "-dir" is to express formal certainties. The aim is to express something in a formal way. You add "-dir" if you want to tell about something in a formal speech. You see it in this task at the official notifications, in the encyclopedical texts etc.

Türkiye (Turkey), başkent (capital city), Ankara (our capital city)

Türkiye+in(genitive) başkent+i(possessive)-->Türkiye´nin başkenti (the capital of Turkey)

Türkiye´nin başkenti Ankara. (The capital of Turkey is Ankara.)

That is a normal sentence. But if you see this knowledge -for example- in an encyclopedia, you probably see in this type:

Türkiye´nin başkenti Ankara´dır.

Because a formal knowledge is being given there.

banka (bank), kapanış (closing, shutdown), saat (1hour, 2clock, 3watch)

kapanış saat+i(poss.)----------------->kapanış saati (the shutdown time)

bankamızın kapanış saati (the shutdown time of our bank)

Bankamızın kapanış saati 17:00´dır. (The shutdown time of our bank is 17:00 o´clock.)

Assume that you saw this on the wall of a bank office. Using "-dir" because it´s an official notification.

böyle (such, like this), kullanıcı (user), bulmak (to find)

böyle bir kullanıcı (a user like this)

bul+ın(passive)+ama(negative ability)+dı(simp.past)-->bulunamadı (it couldn´t be found)

bul+ın(passive)+ama(negative ability)+dı+dır--------->bulunamadıdır

bul+ın(passive)+ama(negative ability)+mış+dır-------->bulunamamıştır (it couldn´t have been found)

Böyle bir kullanıcı bulunamamıştır. (No user like this have been found)

For example, you saw this on Facebook. It´s a formal notification as well.

aile (family), kat (1floor, 2storey), yukarı (upward, upside)

aile kat+mız---------------->aile katımız (our family floor)

yukarı+da(locative)--------->yukarıda (it´s upstairs)

Aile katımız yukarıdadır. (Our family floor is upstairs.)

Imagine that you saw this notification in a restaurant.

Türkiye (Turkey), cumhuriyet (republic), 1923 ((by letters) bin dokuz yüz yirmi üç), kurmak (to found)

Türkiye cumhuriyet+i(possessive)------------>Türkiye Cumhuriyeti (the Republic of Turkey)

1923+de(locative)-------------------------->1923´te (in 1923)

bin dokuz yüz yirmi üç+de

kur+il(passive)+di(simple past)--------------->kuruldu (it was founded)

kur+il(passive)+di(simple past)+dir----------->kuruldudur

kur+il(passive)+miş+dir---------------------->kurulmuştur

Türkiye Cumhuriyeti 1923´te kuruldu. (The Republic of Turkey was founded in 1923.)

this is a normal sentence, it´s not belonged to a special place.

Imagine you saw this knowledge in an encyclopedia:

Türkiye Cumhuriyeti 1923´te kurulmuştur. (The Republic of Turkey was founded in 1923.)

Mısır (Egypt), piramit (pyramid), gizem (mystery), bugün (today), bile (even), korumak (1to protect, 2to keep, 3..4..)

piramitler (pyramids)

Mısır piramitler+i----------->Mısır piramitleri (the pyramids of Egypt)

gizem+leri(poss.)+i(acc.)--->gizemlerini (their mystery (acc.))

bugün bile (even today)

korumak+da(locative)------>korumakta (it is keeping)

Mısır piramitleri, gizemlerini bugün bile koruyor.  (The pyramids of Egypt have been keeping their mystery even today.)

=Mısır piramitleri, gizemlerini bugün bile korumakta.

If you want to tell about this knowledge by a formal speech:

Mısır piramitleri, gizemlerini bugün bile korumaktadır.

yangın (fire (event that fire greatens and burns many places)), zil (bell), gereksiz (unnecessary), kullanmak (to use), ceza (punishment), cezalandırmak (to punish)

yangın zil+i(poss.)----------------->yangın zili (the fire bell)

yangın zil+i(poss.)+i(accusative)--->yangın zilini (the fire bell (acc.))

kullan+an(verbal adjective)+lar----->kullananlar (the ones who use/used)

ceza+la+n+dır+il(pass.)+acak(future)>cezalandırılacak[lar] ([they] he will be punished)

Yangın zilini gereksiz kullananlar cezalandırılacak. (People who use the fire bell unnecessarily will be punished.)

Assume you saw this inside a train or on the wall of an work place. They can be regarded as official notifications, too:

Yangın zilini gereksiz kullananlar cezalandırılacaktır. (People who use the fire bell unnecessarily will be punished.)

Dünya (the Earth, world), Güneş (the Sun), yakın (near, close), üçüncü (third), gezegen (planet)

Dünya, Güneş´e en yakın üçüncü gezegen. (The Earth is the nearest third planet to the Sun.)

If you see this knowledge in an encyclopedia or another texts:

Dünya, Güneş´e en yakın üçüncü gezegendir.

Because a formal certainty is being told in a formal way.