OLDUĞUNU - In conversation on Turkish I have seen this. Which is this tense?  

And something more about this? When would I use it... etc.

It is one of the most used suffix i think.(-dığı / -diği / -duğu etc )

Olduğunu means "that is"

Senin güzel olduğunu biliyorum. --> I know that you are beautiful.

Bu kalemin senin olduğunu bilmiyordum. --> I didnt know that this pencil was yours.

Bildiğini biliyorum. --> I know that you know.

Geldiğini duymadım --> I didnt hear that you come.

Dediğini duymadım. --> I didnt hear what you said.

Yaptığını beğendin mi? --> Do you like what you have done?

EXCELLENT! Thank´s a lot! It was really helpful !

How it looks like sentence like this:
He told that you are stupid.
Senin aptal olduğunu söyledi??
 

Doğru.

WOULD? How it looks in Turkish?  

Quote:nemanjasrb

WOULD? How it looks in Turkish?

You can´t really ask this way.

o.O

Try another way.

Ok. .. In Turkish language,how is building would case? That´s only what I need.