How is (ta) ki used to express purpose?

                 Oturdum ki bir dakika dinleneyim. ´I sat down in order to rest for a while.´

Can I use it in 3rd person:

                 ?Nine oturdu ki bir dakika dinlene/dinlensin. ?

Can I use different persons in the main clause and subclause:

                  ?Hepimiz oturduk ki sen bir dakika dinlenesin. ?

These are the kind of sentences a native speaker wouldn´t prefer to utter. Rather they are the kind you can expect to hear from a non-native speaker (maybe).

I would rather give you how a native would normally say it:

So I can forget about the whole ki thing when talking about purpose. Ok, that´s better. I didn´t like it anyway. My grammar book even though it gave the first example says the meaning of purpose is in the subjunctive, not in the conjunctions.

Thanks for the straight answer, si++.

You mentioned "ta ki ...a kadar" but didn´t give any example.

It´s an used form and means "until". More likely to hear from a native compared to bare "ki" form.

Ali´nin öbür tarafa geçtiğini bilmiyordum ta ki Veli bana soylene kadar.

I didn´t know that Ali passed away until Veli told it to me.

Parkta koştum ta ki yorulana kadar.

I ran in the park until I was tired.

Still they can be said in more natural way:

Veli (bana) söylene kadar Ali´nin öbür tarafa geçtiğini bilmiyordum.

Yorulana kadar parkta koştum.

I don´t have a good example. My grammar book which dates to 1960´s says ta ki or ki alone sometimes introduces subclauses of purpose with subjunctive predicate. My first example was from the book. There is one with ta ki also, it looks complicated but I can copy it here:

o vakit, bir sağa bir sola başvurmağa başlıyorsunuz; ta ki daldığınız bu toprak deryası içinden kendinize bir iz bulup çıkasınız

Anyway, it doesn´t seem to be in the meaning ´not until´ here. The author stresses that the meaning of purpose is in the subjunctive mode, not in ta or ki or anything else.

I have got used to seeing subjunctive forms together with diye and that is what you suggested also, si++. The border between diye and için structures is more difficult to draw though...

Right. Corrected.

I can´t help thinking what ta ki actually brings to this sentence except an Indo-European flavour. Isn´t

                         present participle + DAT + kadar

quite enough to express ´until´?

                        > Veli bana söylene kadar Ali´nin öbür tarafa geçtiğini bilmiyordum.

My exact thoughts! It´s just a Persianism in Turkish.