Learn Hardcore Turkish: Biletler bitmesin diye şimdiden iki tane ayırttık. - We reserved two tickets already so they won’t run out.

Here diye introduces a purpose clause: "so that / in order that." It links the intention (Biletler bitmesin) to the action. For reasons you’d use çünkü or -dığı için; for purpose, use diye (or için with a verbal noun).

What grammatical form is bitmesin?

It’s the 3rd person negative optative/subjunctive: bit- (finish/run out) + -me- (negation) + -sin (3rd person optative) → "may it not run out." With diye, Turkish often uses this form to express purpose: "so that it doesn’t …"

Shouldn’t it be bitmesinler since biletler is plural?

No. With inanimate plural subjects, Turkish usually keeps the verb in 3rd singular. Bitmesinler is possible but sounds marked or personifying; Biletler bitmesin is natural.

Why isn’t it biletleri (accusative)?

Because biletler is the subject of the subordinate clause (Biletler bitmesin). Subjects are in the nominative. Accusative marks a definite direct object, which this is not.

Why is biletler plural? Could I say bilet bitmesin?

The plural refers to tickets as a stock/category. Bilet bitmesin is understandable but less natural; Turkish prefers the bare plural for general reference to countable items.

Can I use için instead of diye?

Yes: Biletlerin bitmemesi için şimdiden iki tane ayırttık. Here the clause is nominalized (bitmemesi "its not running out") and the subject takes genitive (biletlerin). With diye, you keep a finite clause and no genitive.

What does şimdiden add compared to şimdi or önceden?

şimdiden: "from now; in advance" relative to a future event (very common here).
şimdi: "now" (right at this moment), without the "in advance" nuance.
önceden: "beforehand/previously," often interchangeable with şimdiden in this context.

What does tane do in iki tane?

tane is a general classifier ("pieces/items"). After numbers it’s optional and colloquial. You can say iki (if the noun is clear), iki bilet, or iki tane bilet. In formal contexts, adet can replace tane.

Is it okay to say just iki tane without repeating bilet?

Yes. The noun is recoverable from context, so iki tane = "two of them." If you want to be explicit, use iki bilet or iki tane bilet.

What’s the difference between ayırmak and ayırtmak, and why ayırttık?

ayırmak: to separate, set aside, allocate.
ayırtmak: to have something set aside for you → to reserve/book (tickets, table, room). For booking tickets, use ayırtmak. Past 1pl is ayırt-tı-k → ayırttık.

Why the double tt in ayırttık? How do I pronounce it?

The stem ends with t (ayırt-) and the past suffix is -tı-, so you get t + t → tt: ayırt + tı + k → ayırttık. Pronounce with a brief geminate

Could I use aldık instead of ayırttık?

You can, but it changes the meaning:

ayırttık: we reserved/asked to have them set aside (may or may not be paid).
aldık: we bought them.

How flexible is the word order? Can the diye clause move?

Quite flexible. Examples:

Şimdiden, biletler bitmesin diye iki tane ayırttık.
İki tane, biletler bitmesin diye şimdiden ayırttık.
İki tane ayırttık, biletler bitmesin diye. (more spoken) Position before ayırttık is typically in focus.

Could I say this with -meden/-madan or use tükenmek?

Yes:

Biletler bitmeden/tükenmeden şimdiden iki tane ayırttık. ("before the tickets run out")
Biletler tükenmesin diye şimdiden iki tane ayırttık. tükenmek = "to be exhausted/run out," common with stock.

How do I say "two of the tickets" (definite) instead of just "two"?

Use a partitive structure with accusative:

Biletlerden ikisini ayırttık.
Or: İki tanesini ayırttık. The -sini marks "two of that specific set."

Why is the purpose clause present/optative (bitmesin) while the main verb is past (ayırttık)?

The optative expresses intended outcome, independent of the main clause tense. We acted in the past so that, from that point onward, the tickets would not run out. Turkish uses -sin for this purpose meaning even when the main clause is past.