What is the standard for using apostrophes in Turkish?

For instance is it more correct to say İstanbula or İstanbul´a

also denizde or deniz´de?

Much appreciated,

The whole standard is hard and long to ezplain, but

ISTANBUL´A and DENİZDE are correct.

If I won´t be dead or something, I´ll post something about this tomorrow, hopefully. But for now, know that we use it after proper names mostly (names of people, animals, places, organizations, etc.).

And AlphaF gave you the right answer for your example already.

In Turkish -Apostrophe- serves for the following purposes;

1. When proper names take possessive or case suffixes ;

Atatürk’ün  ---> Atatürk´s

Türkiye´mizin  en güzel şehri  ---> the most beautiful city of  our Turkiye.

Fatih Sultan Mehmet’e

Yunus Emre’yi

Cebrail’den

Samanyolu’nda

2.   In shortened  proper nouns.

Istanbul  Boğazı  ---> Istanbul  Strait.

when we drop " İstanbul" we are left with "Boğaz´ and we refer " Istanbul strait" when we just use "boğaz´. Therefore it is still a ´ proper name´ and takes apostrophe. 

Boğaz´dan ---> From the straight [in fact we refer " Istanbul Boğazı".

3. When we refer a certain law, we use apostrophe.

Bu  kanun´un 17. maddesinin  c  bendi…. 

 Article 17th of this law……..

4. When  compound noun "Avrupa Birliği"  takes suffix ;

Avrupa  Birliği´ne üye ülkeler…..

The member state of  European Union….

Attention 1 !  If  a proper name with 3th singular personal marking takes another  different suffix, then we don´t use an apostrophe.

   Boğaz Köprümüzün güzelliği,----> The beauty of  Bosphorus Bridge.

Attention 2 !  The suffixes that are attached onto  institution, commission, assembly and business names can not be separated with apostrophe.

   Türkiye Büyük Millet Meclisine  --> to  the Turkish National Assembly

    Türk Dil Kurumundan  --->  From the Turkish Language Institution..

    Mavi Köşe Bakkaliyesinden  ---> From the Blue Corner shop…

Attention 3 !  Words like " Başbakanlık" [the prime minister´s office], "Rektörlük" [ The office of  a Rector] follows  with a suffix that starts with a vowel , they changes into "Başbakanlığa", Rektörlüğe" and they don´t take an apostrophe.

Attention 4 !   The derivational suffixes, plural suffix and the following suffixes that comes onto proper nouns, can not be separated by an apostrophe.

Türklük  ---> Turkishness   [  -lük is derivational suffix]

Avrupalı ---> European [person]     [ -lı  is derivational suffix]

Hristiyanlıktan  ----> From Christianity 

Mehmetler  ----> Mehmets     [ -ler  is plural suffix]

5.  When person names take  titles, the following suffixes are separated by an apostrophe.

Ahmet  Bey´e  ----> to  Mr. Ahmet

Ayşe  Hanım´dan ---> From Ayşe Hanım

Enver  Paşa´ya   ---> to Enver Pasha

6.  Apostrophe is also used to separate the suffixes that comes on to the abbreviations. 

TBMM’nin    [ TBMM = the Turkish National Assembly] 

TDK’nin        [ Turkish Language  Institution]

 BM’de           [ BM = United Nations]

 ABD’de         [ ABD = United States of  America]

 TV’ye            [ TV =  Television]

7. The suffixes that come onto the numbers are separated by an apostrophe.

1985’te  ----> In 1985

 8’inci madde  ---> 8th Article 

 2’nci kat  ----> 2th floor

 657’yle   ----->  with 657

8.  It is used with  dates that refer a certain month and day.

Başvurular 17 Aralık’a kadar sürecektir  ---> Applications will continue until the 17th of December.

 8. It is used to indicate "the sound reductions" in statements. These type of reductions are seen especially in poems. 

Güzelliğin on par’etmez

Bu bendeki aşk olmasa (Âşık Veysel)

Your  beauty worths no value

If  I didn´t have this love for you

par’etmez  ---> in this word couple, the "a" sound dropped. It is normally " para  etmez "  [worth not a penny]

9. It is used to separate the letters from suffixes.

a’dan  z’ye kadar   ----> from  a   to   z 

Thanks Tunci for your very comprehensive answer. It makes so much more sense now