But i already have the suffix "din" that gives this meaning so why to use "ki"?

In the lesson called vowel rules, at the end there is this sentence

kedi-in-ki-den -> kedi-n-in-ki-n-den -> kedininkinden (from the cat´s)

´in´ is a suffix that gives genitive meaning and ´den´ is a suffix means ´from´, what about ´ki´?

Thanx everybody

Thank you all, teşekkür ederim Faruk very helpful
but I'm still not sure about:

Are you going to the club?
Aren't you going to the club?

Were you a doctor?
Weren't you a doctor?

I have difficulties in constructing questions in different tenses, i would be so grateful if someone translate these sentences for me:

1) To Be
Are you a doctor?
Aren't you a doctor?
Were you a doctor?
Weren't you a doctor?

2) Present Tense
Do you go to the club?
Don't you go to the club?

And the last two sentences in the remaining tenses (past,present continuous, and future tenses)...

Herkes selam,

1)When i want to express possession, should i use (benim, senin, onun, bizim, sizin, onalrın) before the noun/adjective then the appropriate suffix? or i may only use the noun/adjective + suffix? and if the later case is correct, how can i differentiate between 'my' and 'I am'?

Example:
To say my beautiful: benim güzelim or just güzelim.
If i say just güzelim how can you know whether i mean 'my beautiful' or 'I am beautiful'?

2)How to say 'you are my friend'?

Thanx.

Herkes teşekkürler

Thank you very much caliptrix, but forgive me i just wanna be sure about it...

You mean that i may use ler or dirler for formal speech or i may not use them at all in informal one.

for example:
Onlar güzeller.
Onlar güzeldirler.
Onlar güzel.

Are all of the above correct?

çok teşekürler.

Hello everybody,
I'm little confused with verb to be endings, in this lesson
Pronouns 1
when i say they are beautiful: Onlar güzel-ler. --> Güzeller.
while in another lesson
The simple present form (aorist) of the verb 'to be'
when i say they are English: Onlar İngiliz(dirler).
Somebody please tell me when to use the ending (ler) or (dirler).
Thanks a lot.