How do you express in Turkish the equivalents of those English relative clauses where the relative pronoun refers to the whole prededing sentence, like

         They boy had done his homework this time which was a surprise for the teacher.

         He never called again which made me sad ?

Make use of noun clauses:

The boy had done his homework this time = Çocuk bu sefer ödevini yapmıştı.

Çocuğun bu sefer ödevini yapması = The boy´s having done his homework this time

Çocuğun bu sefer ödevini yapması öğretmeni şaşırttı = The boy´s having done his homework this time was a surprise for the teacher.

He never called again = (O) tekrar hiç aramadı.

(Onun) tekrar hiç aramaması = His never calling again

(Onun) tekrar hiç aramaması beni üzdü = His never calling again made me sad.

Now this is the natural Turkish way. But you may make use of Persian ki and it would have the same structure.

They boy had done his homework this time which was a surprise for the teacher.

Çocuk bu sefer ödevini yapmıştı ki bu öğretmeni şaşırtı.

         He never called again which made me sad ?

Tekrar hiç aramadı ki bu beni üzdü.

And this is (or has been) for us Turks (or at least for me) an helper to decode the English sentences like these. I would first translate them this way to Turkish and then turn them into natural one after understanding the meaning.

Thanks. It seems that most relative clauses are translated into Turkish using participles and personal participles plus sentence elements which are changed into adjective attributes. In these examples only infinitives are brought into use.

Interestingly, as your post shows, this question seems to be a backdoor entrance into understanding some features on the use on Persian ki.

          The men to whom the President spoke were patriots.

          We saw the man who my father bought the car from.

When the relative pronoun in an English sentence is the object of a preposition and they together act as an adverbial I can´t think of another way to say it in Turkish but with the help of personal participles. The result, though, in my eyes looks ambiguous:

          Cumhurbaşkanın konuştuğu adamlar çok vatanperverdirler. (talked ´to them´ or ´with them´ or ´about them´?)

And this is even worse:

          Babamın arabayı satın aldığı adamı gördük. (bought ´from the man´ or ´with the man´ or ´for the man´?)

Just like there is something missing but I can´t understand what it is.

          Babamın arabayı satın aldığı adamı gördük.

We saw the man from who my father bought the car.

          Cumhurbaşkanın konuştuğu adamlar çok vatanperverdirler

The men with whom the president talked [spoke] were patriots/

as you know in Turkish we say " Onunla konuştum " ---> Onun +ile [with him ]

                                          " Seninle konuştum " ---> Senin + ile [with you ]

just a tiny bit correction for " vatanperverdirler " as the statement is not past tense, we better say  are patriots instead of  were patriots.

this is not related to your question but its just a different aspect of  it.

Babamın arabayı satın aldığı adamı gördük.

 Cumhurbaşkanın konuştuğu adamlar çok vatanperverdirler

if you want to make these sentences "Subject Relativization with -An "

Babama        arabayı            satan             adamı              gördük.

[to my father]   [the car]  [who sells,sold]    [ the man ]      [we saw]

We saw the man who sold the car to my father.

Başkanla               konuşan          adamlar      çok        vatanperverdirler.

[with president ]    [who spoke]    [the men]    [very]      [patriots]

The men who spoke with president are very patriots.

Note ; "dir" suffix gives the "definite " meaning in the word. we are definetely sure that they are patriots. Its used for definite facts such as ;

Ankara Türkiye´nin başkentidir. ---> Ankara is the capital city of Turkey.

Do you mean there is no problem with Babamın arabayı satın aldığı adamı gördük even though nothing implies that it was from the man, not for him or with him for instance? It would be correctly understood without ambiguity  -  in its own context of course?

I can see that the -an-participle structures which you suggested express the same meaning and they look more clear to me but introduce another point of view.

Yes the natural way is to use -dik (non-future) or -ecek (future). I have posted some detaied mesages in the past about it. See for example:

http://www.turkishclass.com/forumTitle_49431

http://www.turkishclass.com/forumTitle_49700

And you may try Persian ki (and hani) approach here as well:

          The men to whom the President spoke were patriots.

Adamlar ki onlara başkan konuştu vatanseverlerdi.

Adamlar hani onlara başkan konuştu vatanseverlerdi.

          We saw the man who my father bought the car from.

Adamı ki ondan babam araba satın aldı gördük.

Adamı hani ondan babam araba satın aldı gördük.

Yes we understand such things from context:

Gittiğimiz eve = the house to which we are going (to is dropped)

Geldiğimiz ev = the house from which we are comning (from is dropped)

Yediğimiz restaurant = the restaurant at which we eat (at is dropped)

Ankara´ya gittiğimiz kız = the girl with whom we went to Ankara (with is dropped)

Denize atladığım adam = The man for whom I jumped into the sea (for is dropped)

Denize atladığım adam = The man with whom I jumped into the sea (with is dropped)