Example:

Otobüse binmeden önce gazete alırım ve otobüste otururken gazeteyi okurum

why I cant use oturunca

Otobuse binmeden once gazeteyi alirim ve yerime oturunca gazeteyi okurum

I cant understand..still..this is the key of an exercise form Teach  Yourself Turkish; maybe some of keys are wrongthere...

Your problem is with the phrase "otobuse oturunca"....in correct Turkish, people rarely sit on the buses - if and when they do, they do not read newspapers there; they sit in their seats in the busses to read the paper.

ok..thank you..but please I have another one

Sabahleyin çalışırken üç veya dört bardak çay içerim

why is not...çalışınca...

what is the reason for using one or another

çok teşekkür ederim

The alternative you suggest is possible...but the two sentences will mean different things.

1. Sabahleyin calisirken uc veya dort bardak cay icerim (I always have 3 or 4 cups of tea while I am working - tea helps me keep my concentration - in the mornings.)

2. Sabahleyin calisinca 3 veya 4 bardak cay icerim ( I always have 3 or 4 cups of tea if -I work in the mornings- working makes me so thirsty.)

In the first sentence the act of having tea is while the work is in progress. In the second sentence, the tea may be before, during or after the work session.

so..as I can understand...I must use -rken form for an in progress action and -ince form for an ended action..hmmm..I think I got it

çok sağol

my best regards

Your understanding is basically correct....smart young lady !

not so youngbut smart certainly(or..hard working..hmm..bilmem)

...oturunca: when i sit...

...otururken: while i am sitting...

Questions

Oturursam ?

Oturabilirsem?

Oturamazsam ?

Oturmaliysam?

Oturmamaliysam ?

any native speaker edit, i am not sure.

I agree with you 

More questions (some simple, some advanced and complicated)

Oturabileceksem ?

Oturacaksam ?

Oturabilsem ?

Oturmamaliydimsa ?

Oturabilemeseydim ?

Oturtulurum?

Oturtulamam ?

Oturmamaliymisim ?

 my try

Almost perfect....well done!

hadi ya which ones are wrong ,give us a glue

glue??      

maybe a clue

opps

can this "ken" be applied to the future tense? e.g. "Kompartımana dönecekken, koridorun sonundaki vagon kapısı açıldı;" 

I cannot comment on your example, but one of my books says that you can use it following future tense suffixes.

If you ask "can -ken be added to -cek", the answer is "yes". 

cek + ken combination is possible..

yiyecekken, iyileşecekken, oturacakken, etc

It is combined gerund.. and used as adverb of time and adverb of manner..

The main verb (predicate ) is always past (as in your and henry´s examples )

Let´s see it in a joke:

paddy x man, paddy y man and paddy z man find a wishing slide one day. paddy x man goes to the top and says "Gold!" and slides down and lands in a pile of gold. 

paddy y man went up next and says "million pounds!" and slides down and lands in a million punds!

paddy z goes up and when he was just about to sit and slide (=tam oturup kayacakken) he slipped and went flying down yelling "Sh**!" 

***

If you ask, "can -ken be applied to the future tense", the answer is again "yes".

The combination for future tense is er+ken..

giderken görürsün, alırken bakarsın, kullanırken farkeder..

or

giderken göreceksin, alırken bakacaksın, kullanırken farkedecek.. 

***

er+ken can be used with all tenses:

alırken görmüş

incelerken farketti

dururken biniyor

giderken ağlayacak

severken aldatmıştı

okurken ağlar

etc.