Hi there!

I have a question about possessive case

in my textbook)

Dünyanın ülkesinde,

this sentence, I dont know why ülkesi´n´de

why ´n´ is there??

in my univ I study possessive

something+(n)In + something + I(s)

so I think that

Dünya+(n)ın  +  ülke+(s)I + DA... = Dünyanın ülkeside

what is diffirent Dünyanın ülkesinde and Dünyanın ülkeside??

And why am I wrong??

Quote from "Turkish An Essential Grammar" by Goksel and Kerslake (which is a great book for Turkish grammar and explanations, especially if you are seriously studying Turkish):

Irregular changes in stems and suffixes

´n´ appears after certain suffixes or words when these are followed by a case suffix, -ce or -ler (except -(y)le). Note that ´n´ is inserted whether the suffix begins with a vowel or a consonant.

1) at the end of pronouns

o (he/she/it/that) examples:

ona = to him/her/it/that

onsuz = without him/her/it/that

bu (this) examples:

buna = to this (one)

bunlar = these

şu (this/that) examples:

şuna = to this/that (one)

şunca = this much

2) at the end of the reflexive pronouns kendi, kendisi and kendileri

kendine = to him/herself

kendisince = in his/her own view

3) at the end of reciprocal pronouns birbiri and birbirlereri

birbirine = each other (accusative)

birbirinden = from each other

4) at the end of 3rd person possessive form -(s)i

bahçesini = his/her garden (accusative)

bahçesinde = in his/her garden (locative)

5) at the end of the pronominal form -ki

evdeki = the one at home, evdekinde = on the one at home

Dünyanın ülkesinde is the only correct form. This is just something you have to learn. Hope the quoted text helps you in your studies. 

Just as a side note, this additional letter -n- is called "pronominal n". Although there are different theories about how it originated, its origin is not exactly known.

Hi,
Could you please write whole sentence? If I know the sentence, I can give more accurate answer.

As a text "Dünyanın ülkesinde," or "Dünyanın ülkeside," is not exact true expression.

These should be "Dünyanın en güzel/hiçbir/ne karanlık/en kuzey ülkesinde," or "Dünyanın ülkesi de,".

1-The suffix "n" indicates possession. It means that the country belongs to the world. For instance "masanın ayağı kırıldı." "The table leg was broken." Firstly, the leg is belong to table. Secondly, it was broken.

2-In Turkish, we have two type of "de".
"Sizin  dükkan da satıldı." Its means "every shop was sold to new owner and additionally your shop was sold. (conjunction)
"Sizin dükkanda satıldı." The things was sold in your shop. (possession)

If you want to write "ülkeside", you should write as "ülkesi de".

I hope that the basic explanation helps you. You may sent pm, if you need.