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...