Kaptan olmak için birilerini seçin.

Choose someone to be the captain.

Thank you

Kaptan olmak için birilerini seçin.

Choose someone to be the captain.

Thank you

That sentence is not complete.

Kaptan olmak için birilerini... = ...someone (some people) to be the captain.

Birileri = people, some people, ones

Birilerini = Accusative case

I hope this helps!

(P.S. Can you post the full sentence, please?)

Aurum thank you. The full sentence is "choose someone to be captain". Kaptan olmak için birilerini seçin.

Kaptan olacak birini seçin.

I think it´s:

Kaptan olmak için birini seçin.

But hopefully someone can check...

Kaptan olması için birini seçin. 

Ol-ma-sı. You need possesive sufix -sı.

" Kaptan olacak birini seç. " this is also correct. Just saying.

Though, to be honest, this is more along the lines of "Pick someone who should/will be the captain."

Aurum, gokuyum and denizli thanks so much!

It is? I´m confused

or with: olacağı ?

"kaptan olacak birini" is an adjective phrase. So it is correct. It means "someone who is going to be a captain"

Think of it like this:

Büyükelçi düzeyinde görevden almalar olacağı açıklandı.

It has been announced that there will be dismissals from duties at the level of ambassador.

Since this is a passive structure, the entire part "Büyükelçi düzeyinde görevden almalar olacağı" is the subject of the verb "açıklanmak".

I hope this helps!

Then why are there ´-i´ in the following examples (from "TURKISH: A COMPREHENSIVE
GRAMMAR - Aslı Göksel and Celia Kerslake" page 384)

(23) [gönderecekleri] temsilci (cf. Bir temsilci gönderecekler.)
‘the representative [(whom) they will send]’

(26) [Erol-un] (kendi-si-yle) konuş-abil-eceğ-i birisi (cf. Erol öyle birisiyle
konuşabilir.)
Erol-GEN kendi-3SG.POSS-COM talk-PSB-PART-3SG.POSS someone
‘someone [with whom Erol can/could/would be able to talk]’

gönderecekleri temsilci:"The representative who they will send" - "gönder-ecek-leri"  this is an adjective verb. An adjective verb can have possesive suffix "ex: -leri" in Turkish because it can function as a noun like all adjectives. "-ecek" here is not only a tense it is also adjective verb suffix.

konuşabileceği birisi : "Someone who he can talk to" "konuş-abil-ecek-i" this is an adjective verb too. -ecek has the same function here too. The adjective verb has a possesive suffix -i here .

If you want detailed information, you must study "fiilimsiler".

http://www.turkdilbilgisi.com/fiilimsiler/fiilimsiler.html

So in the sentences above there are possesive suffixes not the definitive (?) suffixes.

(?) I dont know how to call them in English

.