Hi!

regarding the past tense, when shoud I use the form   "oldu" and when "du"

thank you very much!

 Oldu as a phrase can be used as  " Ok, All right "

 oldu o zaman  = Ok, then

Oldu  as  past tense form of the verb "to be" , "to become" ,"to happen" , " to occur", " to take place"

Ahmet doktor oldu ---> Ahmet became a doctor.

Çok kötü bir kaza oldu bugün.-----> A very bad accident happened today.

Düğünleri Istanbul´da oldu  ---> Their wedding took place in Istanbul.

"du" is past tense ending, actually the endings for past tense are "dı ,di, du, dü ". Stem of the Verb takes one of them according to wovel harmony.

In our case it was = Ol [ Verb stem] + du [past tense ending] ---> o ---> u

other examples ;  Gel [to come] + di [ past tense ending] --> e -----> i

                         Al [ to take ] + dı [past tense ending] ---> a -----> ı

                         Gül [ to smile] +dü [past tense ending ] --> ü ---> ü

The general formula of  Past Tense is ;

VERB [STEM] + DI,Dİ,DU,DÜ + PERSONAL ENDING

anladim.

ama bunlardan

araba çok güzel oldu  ve araba çok güzeldi.

nasıl çevirebilirim?

teşekkür ederim!

 araba çok güzel oldu ---->  in this case "oldu" is like " to turn into "

the car turned into much nicer [look,condition]

the car changed into much better [look,condition]

after your car has been modified, repaired, you can say " araba çok güzel oldu "

 araba çok güzeldi --->  The car was very nice

               güzel + di

çok teşekkür ederim!

tunci explained it well...here are some more examples:

dün güzel bir gün oldu - yesterday happened to be a nice day

dün güzel bir gündü - yesterday was a nice day

dün güzeldi - yesterday was nice

düğün istanbul´da oldu - the wedding happened to be (took place) in İstanbul 

düğün İstanbul´daydı - the wedding was in İstanbul

bir kaza oldu - there happened to be an accident

bir kazaydı - it was an accident

ne oldu? - what did happen/ what happened?

neydi? - what was it?

nerede oldu? - where did it happen?

neredeydi? - where was it?

doktor oldu - s/he happened to be (= became) a doctor

doktordu - s/he/it was a doctor

hasta oldu - s/he happened to be (=became) ill

hastaydı - s/he was ill

 örnekleriniz çok faydalı.

çok sağolun!