jīnwǎn wǒ huì mǎidān.

In Mandarin, time words often come near the beginning of the sentence, before the main verb.

So:

• 今晚 我 会 买单。
• literally: Tonight, I will pay the bill.

A very common basic order is:

• Time + Subject + Auxiliary/Modal + Verb

You could think of it as setting the scene first: tonight, then saying who is doing something, then what will happen.

今晚 means tonight.

It is a shorter, very common way to say:

• 今天晚上 = this evening / tonight

So:

• 今晚 = more compact
• 今天晚上 = a little fuller, but same basic meaning in many contexts

In everyday speech, both are natural.

For example:

• 今晚我会买单。
• 今天晚上我会买单。

Both can mean I’ll pay tonight.

Here, 会 means will or be going to.

In this sentence, it shows a future action:

• 我会买单 = I will pay the bill

But 会 can have other meanings in other contexts, such as:

• know how to / be able to
  
  • 我会说中文。 = I can speak Chinese.
• be likely to
  
  • 明天会下雨。 = It will probably rain tomorrow.

So 会 is a flexible word. In 今晚我会买单, the most natural interpretation is future intention: I’ll pay tonight.

Technically, 会 can sometimes mean know how to, but in this sentence that is not the natural reading.

Because of the context:

• 今晚 = a specific time in the future
• 买单 = a concrete action in a restaurant or similar setting

So 我会买单 here is understood as:

• I will pay the bill
• not I know how to pay the bill

If you wanted to say I know how to pay the bill, that would usually only make sense in a very unusual context.

Yes, literally:

• 买 = buy
• 单 = list, slip, bill, check

But as a fixed expression, 买单 means:

• pay the bill
• pick up the tab

This is a very common colloquial expression, especially in restaurants.

A few related expressions:

• 买单 = pay the bill / pick up the tab
• 结账 = settle the bill / check out
• 付钱 = pay money
• 请客 = treat someone / pay for others as host

So 今晚我会买单 specifically has the sense of Tonight, I’ll cover it.

They are related, but not identical.

• 买单
  = pay the bill / pick up the tab
  Focus: who is actually paying

• 结账
  = settle the bill / check out
  Focus: completing payment, often used by customers calling for the bill or paying before leaving

• 请客
  = treat someone / host and pay
  Focus: generosity or invitation

Examples:

• 今晚我买单。
  = I’m paying tonight.

• 服务员，结账。
  = Excuse me, check please.

• 今天我请客。
  = Today it’s my treat.

So in your sentence, 买单 is a natural choice if the idea is I’ll be the one paying.

No. 今晚我会买单。 is already complete and natural.

Why?

• 会 already marks the action as future
• 了 is not required just because something is in the future

Adding 了 here would usually sound unnatural in this simple sentence.

Natural:

• 今晚我会买单。

Less natural in this context:

• 今晚我会买单了。

So if you want to say I will pay tonight, the version without 了 is the standard choice.

Yes. 我今晚会买单 is also natural.

Compare:

• 今晚我会买单。
• 我今晚会买单。

Both mean basically the same thing.

The difference is mainly emphasis:

• 今晚我会买单。
  slightly foregrounds tonight
• 我今晚会买单。
  slightly foregrounds I

Mandarin often allows some flexibility with time words, as long as the sentence stays clear and natural.

No. Standard written Chinese normally does not use spaces between words.

So this sentence is usually written as:

• 今晚我会买单。

Learning materials often add spaces to help beginners see the words:

• 今晚 我 会 买单。

That is only for study purposes, not normal writing.

It is natural and fairly neutral, but 买单 has a somewhat conversational, everyday feel.

So this sentence sounds very normal in speech, especially in situations like:

• dinner with friends
• a date
• eating at a restaurant

If you wanted something more formal, you might choose a different expression depending on the context. But for everyday Mandarin, 今晚我会买单。 is perfectly natural.