Nếu chuyến bay đến trễ, chúng tôi sẽ ăn tối ở sân bay.

Nếu means if. It introduces the condition.

So the structure is:

Nếu + condition, result

In this sentence:

Nếu chuyến bay đến trễ, chúng tôi sẽ ăn tối ở sân bay.

= If the flight arrives late, we will eat dinner at the airport.

This is a very common Vietnamese pattern for if sentences.

Chuyến bay is the normal Vietnamese word for flight.

• bay = to fly / flying
• chuyến = a trip, journey, or service occurrence

Together, chuyến bay means a particular flight, like an airline flight.

You can think of chuyến here as part of a fixed noun expression, not something you would usually translate word-for-word every time.

Other similar examples:

• chuyến tàu = train trip / train service
• chuyến xe = bus trip / ride / service

Yes. Đến trễ literally means arrive late.

• đến = arrive, come
• trễ = late

So Vietnamese expresses this idea very similarly to English.

You may also hear:

• đến muộn

This means the same thing. Both trễ and muộn can mean late. In many contexts:

• trễ is especially common in southern Vietnamese
• muộn is often more common in the north

Both are widely understood.

Yes, both are possible, but they feel slightly different.

• chuyến bay đến trễ = the flight arrives late
• chuyến bay bị trễ = the flight is delayed / the flight gets delayed
• chuyến bay trễ = the flight is late

So:

• đến trễ focuses on the arrival being late
• bị trễ focuses more on the flight being delayed
• trễ by itself is shorter and more conversational

In your sentence, đến trễ is natural and clear.

Sẽ is a future marker. It often means will.

So:

• chúng tôi sẽ ăn tối = we will eat dinner

Vietnamese does not always need a future marker if the time is already clear from context, so you might also hear:

• Nếu chuyến bay đến trễ, chúng tôi ăn tối ở sân bay.

That can still be understood as future because the whole sentence is about a future possibility. But adding sẽ makes the future meaning more explicit.

Vietnamese often does not need a separate word for then.

English can say:

• If the flight is late, then we’ll eat at the airport.

Vietnamese can simply say:

• Nếu chuyến bay đến trễ, chúng tôi sẽ ăn tối ở sân bay.

However, you can also use thì in the second clause:

• Nếu chuyến bay đến trễ, thì chúng tôi sẽ ăn tối ở sân bay.

Here, thì works a bit like then or a topic/result marker. It is common, but not required.

This is an important distinction.

• chúng tôi = we, but not including the listener
• chúng ta = we, including the listener

So if the speaker is talking about their own group, not the person they are speaking to, chúng tôi is correct.

If the listener is part of the group who will eat at the airport, then chúng ta might be more appropriate.

Example:

• Nếu chuyến bay đến trễ, chúng ta sẽ ăn tối ở sân bay. = If the flight arrives late, we’ll eat dinner at the airport.
  Here, we includes the person being spoken to.

Ăn tối is a very natural way to say eat dinner or have dinner.

• ăn = eat
• tối = evening

So ăn tối literally looks like eat evening, but it functions as have dinner.

Other meal expressions work similarly:

• ăn sáng = have breakfast
• ăn trưa = have lunch
• ăn tối = have dinner

You could also say ăn bữa tối, but ăn tối is shorter and more common in everyday speech.

Ở here means at or in, depending on the place.

• sân bay = airport
• ở sân bay = at the airport

In this sentence, it tells you the location of the dinner.

So the structure is:

ăn tối ở sân bay
= eat dinner at the airport

Vietnamese often uses ở before a place to show location.

Examples:

• ở nhà = at home
• ở trường = at school
• ở sân bay = at the airport

Yes. The given sentence starts with the if clause, which is very common:

• Nếu chuyến bay đến trễ, chúng tôi sẽ ăn tối ở sân bay.

But you can also put the main clause first:

• Chúng tôi sẽ ăn tối ở sân bay nếu chuyến bay đến trễ.

This means the same thing: We’ll eat dinner at the airport if the flight arrives late.

Both are natural. Starting with nếu often puts more emphasis on the condition.

The comma is standard and helpful when the if clause comes first:

• Nếu chuyến bay đến trễ, chúng tôi sẽ ăn tối ở sân bay.

It separates the condition from the result, just like in English.

If the if clause comes second, people often write it without a comma:

• Chúng tôi sẽ ăn tối ở sân bay nếu chuyến bay đến trễ.

In casual writing, punctuation in Vietnamese can be a little flexible, but the comma in the original sentence is normal and correct.

Yes.

You can say:

• Nếu chuyến bay đến muộn, chúng tôi sẽ ăn tối ở sân bay.

This has the same meaning.

Very roughly:

• trễ is often associated more with southern usage
• muộn is often associated more with northern usage

But both are standard and understood across Vietnam.

It is neutral and natural. It works well in everyday speech and normal writing.

A few parts affect the tone:

• chúng tôi sounds a bit more standard than very casual pronouns
• sẽ makes it clear and complete
• ăn tối ở sân bay is perfectly natural everyday Vietnamese

So this is a good model sentence for learners: clear, grammatical, and commonly usable.