how do you say this in turkish,

"Did you write that? It´s really beautiful."

bunu sen mi yazdin? gercekten cok güzel

im not sure about the first part but i think so

I believe it is:

Bunu yazdın mı?
Çok güzel.

But hopefully a native speaker can confirm? Nessah´s may also be correct.

I vote for Nessah´s translation. Few corrections to ortography:

Bunu sen mi yazdın? Gerçekten çok güzel.

But let´s wait for comments from native speakers or experienced learners.

Nessah´s translation is correct. I am shedding tears of joy now

I hope I can understand this.

Is Bunu yazdın mı? grammatically correct? Would that mean more like "did you write it yet"?

And Bunu sen mi yazdin? is empasizing the you i.e. Was it you who wrote this?

Yes, I think you are right. However, I think that in original sentence there is some emphasis on actually asking if "it is you who wrote that" - someone thinks that "this" is beautiful, wants to share his/her opinion with author and asks if "you" are the author.

Below are my learner´s thoughts on this - I may be not right here, so better wait for other forum members to check it.

For example, if question was:

Did you write the letter? (for example, letter which you were supposed to write)

Then I think that this can be translated like you did: Mektubu yazdın mı ?

But, in this case:

I disagree with author of this letter. Did you write it?

I think it could be translated like this: Bu mektubun yazarıyla aynı görüşte değilim. Bu mektubu sen mi yazdın?

You can emphasize any word putting question word after it in Turkish. So you can ask that question in many ways?

Bu mektubu sen mi yazdın?

Sen bu mektubu mu yazdın?

This one is interesting because in Turkish the word before verb is emphasised but this question word emphasise the verb too coming after it. So two things are emphasised

Sen bu mektubu yazdın mı?