Nếu trời mưa, chúng tôi sẽ ở lại khách sạn.

Nếu means if.

In this sentence, it introduces the condition:

• Nếu trời mưa = If it rains
• chúng tôi sẽ ở lại khách sạn = we will stay at the hotel

This is a very common Vietnamese pattern:

• Nếu + condition, result
• Nếu trời mưa, chúng tôi sẽ ở lại khách sạn.

You will also often see nếu ... thì ..., where thì marks the result clause:

• Nếu trời mưa, thì chúng tôi sẽ ở lại khách sạn.

In everyday speech, thì is often omitted, as in your sentence.

Trời mưa literally looks like sky/heaven rains, but it simply means it is raining or it rains.

• trời = sky, weather, heaven
• mưa = rain / to rain

Vietnamese often uses trời in weather expressions:

• trời mưa = it’s raining
• trời nắng = it’s sunny
• trời lạnh = it’s cold

In casual speech, people also very often say just:

• Nếu mưa, chúng tôi sẽ ở lại khách sạn.

So both are natural, but trời mưa sounds a bit fuller and very standard.

Here, mưa functions like a verb, meaning to rain.

Vietnamese words often do not change form the way English words do, so one word can serve different roles depending on context.

For example:

• mưa = rain (noun)
• trời mưa = it rains / it is raining (verb-like use)

So in Nếu trời mưa, mưa is best understood as to rain.

This is a very important Vietnamese distinction.

• chúng tôi = we, but not including the person being spoken to
• chúng ta = we, including the person being spoken to

So in your sentence:

• chúng tôi sẽ ở lại khách sạn = we will stay at the hotel, but you are not part of that we

If the speaker wanted to include the listener, they would say:

• chúng ta sẽ ở lại khách sạn

English uses we for both ideas, but Vietnamese makes the distinction clearly.

Sẽ is a future marker. It often means will.

So:

• chúng tôi sẽ ở lại khách sạn = we will stay at the hotel

In this conditional sentence, sẽ helps show that the staying happens in the future if the condition comes true.

However, Vietnamese often omits future markers when the meaning is already clear from context. So you may also hear:

• Nếu trời mưa, chúng tôi ở lại khách sạn.

That can still be understood as If it rains, we’ll stay at the hotel, especially in conversation.
Using sẽ just makes the future meaning more explicit.

Ở means to stay, live, be at.
Ở lại means to stay behind, remain, stay on.

So:

• ở khách sạn = be/stay at the hotel
• ở lại khách sạn = stay at the hotel / remain at the hotel

In this sentence, ở lại gives the sense of remaining there instead of going somewhere else.

For example:

• Nếu trời mưa, chúng tôi sẽ ở lại khách sạn. = If it rains, we’ll stay at the hotel.

This sounds a bit like: we had some other possible plan, but if it rains, we’ll remain at the hotel.

Vietnamese does not use articles like a, an, and the the way English does.

So:

• khách sạn = hotel / the hotel / a hotel

The exact meaning depends on context.

In your sentence, khách sạn is naturally understood as the hotel because the situation probably already makes clear which hotel is being discussed.

If needed, Vietnamese can be more specific in other ways, for example:

• khách sạn này = this hotel
• khách sạn đó = that hotel

Yes. Vietnamese can put the result first and the condition second, just like English sometimes can.

Your sentence:

• Nếu trời mưa, chúng tôi sẽ ở lại khách sạn.

A reordered version:

• Chúng tôi sẽ ở lại khách sạn nếu trời mưa.

Both are natural and mean the same thing:

• If it rains, we will stay at the hotel.
• We will stay at the hotel if it rains.

The version with nếu at the beginning is often a little clearer for learners because the condition is announced immediately.

The comma is helpful, but in informal writing you may see Vietnamese without it.

• Nếu trời mưa, chúng tôi sẽ ở lại khách sạn.
• Nếu trời mưa chúng tôi sẽ ở lại khách sạn.

Both can appear, especially in casual contexts.
The comma simply makes the division between condition and result easier to read.

In careful writing, using the comma here is a good idea.

Yes. You can say:

• Nếu trời mưa, thì chúng tôi sẽ ở lại khách sạn.

Here, thì marks the main result: then.

So the structure becomes:

• Nếu ... thì ... = If ... then ...

But in natural Vietnamese, thì is very often omitted when the meaning is obvious. That is why your original sentence sounds perfectly normal.

A good way to break it down is:

• Nếu = if
• trời mưa = it rains / it is raining
• chúng tôi = we (not including you)
• sẽ = will
• ở lại = stay/remain
• khách sạn = hotel / the hotel

So the grammar pattern is:

• Nếu + clause, subject + sẽ + verb phrase

In full:

• Nếu trời mưa, chúng tôi sẽ ở lại khách sạn.

This is a very useful model you can reuse:

• Nếu bận, tôi sẽ gọi lại sau.
  If I’m busy, I’ll call back later.

• Nếu có thời gian, chúng ta sẽ đi ăn.
  If we have time, we’ll go eat.

It is neutral and standard.

Nothing in it is especially formal, but it is polite and natural.
The most notable choice is chúng tôi, which is standard and clear.

In more casual speech, depending on region and relationship, speakers might use other words for we, such as:

• tụi tôi
• bọn tôi
• tụi mình (context-dependent)

But chúng tôi is a very safe and useful form for learners.

A few important pronunciation points:

• nếu — rising tone on nếu
• trời — falling-rising contour; learners often find this one tricky
• mưa — level tone
• chúng — has the ng ending
• tôi — sounds roughly like toy, but not exactly English toy
• sẽ — rising creaky tone in the South / glottalized contour depending on accent
• ở — be careful with the vowel; it is not like English o
• lại — falling tone
• khách sạn — two words; sạn has a heavy/glottalized tone

If your goal is to be understood, focus especially on:

1. saying chúng tôi clearly,
2. not dropping sẽ,
3. pronouncing ở lại as two separate syllables,
4. keeping the tones distinct in nếu, trời, and lại.

Vietnamese pronunciation varies by region, so you may hear somewhat different tone realizations in northern vs. southern speech.