chúfēi péngyou mǎidān, fǒuzé wǒmen píngtān.

除非……否则…… is a very common pattern meaning unless ..., otherwise ....

So:

• 除非朋友买单 = unless a friend pays the bill
• 否则我们平摊 = otherwise, we split it

It sets up a condition:

• if the condition in 除非 happens, the result in 否则 does not happen
• if the condition does not happen, then the 否则 part happens

A very natural English translation is:

Unless a friend picks up the tab, we’ll split the cost.

In Mandarin, 除非 already contains the idea of unless by itself, so you do not need a separate word for if.

Compare:

• English: Unless a friend pays, ...
• Chinese: 除非朋友买单，...

So 除非 is not just a simple vocabulary word; it introduces a whole condition.

否则 is often used with 除非 to make the contrast very clear, but in some contexts it can be omitted if the meaning is obvious.

With 否则:

• 除非朋友买单，否则我们平摊。
• Unless a friend pays, otherwise we split it.

Without 否则:

• 除非朋友买单，我们就平摊。
• still understandable: Unless a friend pays, we’ll split it.

That said, 除非……否则…… is a very standard and natural pairing, so learners should get used to seeing it.

买单 literally looks like buy the bill, but in real usage it means:

• to pay the bill
• to pick up the tab
• sometimes more generally, to pay

In a restaurant or group meal context, it is extremely common.

Examples:

• 谁买单？ = Who’s paying?
• 今天我买单。 = I’ll pay today.

So in your sentence, 朋友买单 means a friend pays for everyone.

Chinese often leaves nouns more general than English does.

朋友买单 literally says friend pays the bill, but it naturally means something like:

• a friend pays
• one of our friends pays
• a friend picks up the tab

Chinese does not always need an article like a or the, and it often omits possessives like my/our when they are not important.

So 朋友 here is general and context-based, not ungrammatical.

平摊 means to split evenly or to divide the cost equally.

In this sentence:

• 我们平摊 = we split the bill evenly

It is close in meaning to AA制, but they are not exactly the same in usage:

• 平摊 is a verb: to split the cost
• AA制 is more like a noun/expression meaning going Dutch or splitting the bill system

Examples:

• 我们平摊吧。 = Let’s split it evenly.
• 今天吃饭AA制。 = Let’s go Dutch today.

So here, 平摊 is the better choice because it directly describes what we do.

Because 我们 is the subject, and Mandarin usually follows Subject + Verb order.

So:

• 我们平摊
• we split it

This is normal word order.

The full sentence structure is:

• 除非
  • condition
• 否则
  • subject + verb

So:

• 除非朋友买单，
• 否则我们平摊。

Yes, the object is understood from context.

平摊 often refers to:

• the bill
• the cost
• the expense

Chinese frequently omits objects when they are obvious. Since 买单 already tells us this is about paying for a meal or bill, 我们平摊 naturally means:

• we split the bill
• we split the cost

If you wanted to be more explicit, you could say:

• 否则我们平摊饭钱。 = Otherwise we split the meal cost.
• 否则我们平摊账单。 = Otherwise we split the bill.

除非 is strongly tied to the idea of an exception, so it often appears in sentences that English would express with unless.

It introduces the only condition under which something different happens.

For example:

• 除非下雨，否则我们去。
• Unless it rains, we’re going.

So yes, it usually carries that exception / only if not logic.

A useful way to think about it:

• 除非 A，否则 B
• = If not A, then B
• = Only if A, not B

In your sentence:

• 除非朋友买单，否则我们平摊
• = If a friend doesn’t pay, we split it
• = Only if a friend pays do we not split it

Yes. The given sentence is correct, but in casual conversation people might say things like:

• 除非朋友请客，不然我们就平摊。
• Unless a friend is treating us, we’ll split it.

or

• 除非有人买单，不然我们AA。
• Unless someone pays, we’ll go Dutch.

A few notes:

• 不然 is a more conversational alternative to 否则
• 请客 often means to treat others / pay for everyone
• AA is common in speech for splitting the bill

Still, the original sentence is perfectly good and clear, especially for learning the grammar pattern.

It sounds neutral and natural, with slightly textbook-like clarity because of the full 除非……否则…… structure.

Word by word:

• 除非 = standard written/spoken
• 否则 = a bit more formal than 不然
• 买单 = very common in everyday speech
• 平摊 = normal, practical, slightly matter-of-fact

So overall, it is grammatical and natural, but a real conversation among friends might more often use:

• 不然
• AA
• 请客

instead of the more neatly structured textbook phrasing.

A natural rhythm is:

除非朋友买单，/ 否则我们平摊。

Pinyin: Chúfēi péngyou mǎidān, fǒuzé wǒmen píngtān.

A few pronunciation notes:

• 朋友 is often pronounced more lightly in real speech, roughly like péngyou
• 我们 is also often lighter in the second syllable: wǒmen
• pause slightly at the comma after 买单
• stress the contrast between 除非 and 否则

So the sentence should sound like two balanced parts:

• unless ...
• otherwise ...