Learn Hardcore Turkish: Neden olmasın, zaten hava çok güzel. - Why not; the weather is already very nice.

It literally means Why should it not be? or Why shouldn’t it happen?
Breakdown: neden = why; ol- = be/happen; -ma- = negative; -sın = 3rd-person optative/imperative (“let it (not) be”). So ol-ma-sın = “let it not be,” and the whole phrase is a set way to say “Why not?”

Why isn’t the yes/no question particle mi/ mı/ mu/ mü used here?

Because neden already makes it a wh-question. Mi is for yes/no questions. You could say Olmasın mı? (“Shouldn’t it be?”) to seek confirmation, but it’s not the same as the idiomatic, agreeable Neden olmasın?. Avoid Neden olmasın mı?, which is awkward.

Can I replace neden with niye or niçin?

Yes. Niye olmasın? is more colloquial; Neden olmasın? is neutral; Niçin olmasın? is a bit formal/literary. All mean “Why not?”

What is the subject of olmasın? What is the “it”?

Turkish often leaves the subject implicit. Olmasın is 3rd person and refers to the proposed action or idea from context (e.g., “sit outside,” “go out,” “have coffee”). So it’s an impersonal “it.”

What does zaten mean here?

Zaten means “anyway,” “as it is,” or “after all.” It presents a reason that’s already known or obvious. It’s not primarily the temporal “already”; for “already (by now)” Turkish more often uses çoktan (e.g., Çoktan geldim).

Where can zaten go, and does position change the nuance?

Zaten hava çok güzel. Fronted, it frames the whole clause as background info (“Anyway, the weather is great.”).
Hava zaten çok güzel. Emphasizes that the weather, specifically, is already nice.
Hava çok güzel zaten. Adds it as an afterthought.
All are grammatical; the choice is about emphasis and flow.

Why is there no verb “to be” in hava çok güzel?

In Turkish, with adjectives (and with nouns in many cases) the present 3rd-person copula is zero. So Hava çok güzel literally “Weather very nice” = “The weather is very nice.”

What does çok add to güzel?

Çok intensifies: çok güzel = “very nice”/“great.” Without çok, güzel is simply “nice.” Note pek is rarely used in positive statements nowadays; it commonly appears with negatives (e.g., pek güzel değil = “not very nice”).

Could I use a different adjective instead of çok güzel?

Yes: harika, şahane, müthiş, mükemmel (stronger praise, “wonderful/great/perfect”), or colloquial süper. For weather, Hava harika or Hava çok güzel are both very natural.

Is the comma correct in Neden olmasın, zaten hava çok güzel?

Yes. Turkish often allows a comma to link closely related clauses. You can also write two sentences: Neden olmasın? Zaten hava çok güzel. A semicolon is possible but feels formal.

What’s the difference between olmasın, olsun, and olur?

Olsun: 3rd-person optative “let it be” → acceptance/permission (“OK then”).
Olur: aorist “it works/it’s OK/it happens” → very common acceptance.
Olmasın: “let it not be” (negative), but in the fixed phrase Neden olmasın? it conveys positive acceptance (“Why not?”). Used alone, Olmasın is a refusal (“Let’s not”).

Does Neden olmaz? mean the same as Neden olmasın?

No. Neden olmaz? = “Why doesn’t it work/why wouldn’t it be possible?” It seeks an explanation and can sound doubtful or complaining. Neden olmasın? signals openness/approval: “Why not!”

What about Neden olmasın ki? What does ki do?

Adding ki gives emphasis, a touch of “indeed” or “come on now”: Neden olmasın ki? ≈ “Why not indeed?” It can sound warmer or more persuasive.

Any tips on pronunciation and intonation?

ç in çok is “ch,” ü in güzel is a front-rounded vowel (like French “u”), ı in olmasın is the dotless back “uh,” and h in hava is pronounced.
Intonation: Neden olmasın typically rises at the end (friendly, inviting). Pause briefly at the comma, then zaten hava çok güzel falls at the end.