Trời hôm nay nóng.

Vietnamese often does not use a copula like English is/are in simple descriptive sentences.

So:

• Trời hôm nay nóng = literally something like Sky/weather today hot
• Natural English: The weather is hot today / It’s hot today

In Vietnamese, an adjective like nóng can directly function as the predicate, so you do not need là here.

• Correct: Trời hôm nay nóng.
• Not natural for this meaning: Trời hôm nay là nóng.

Trời can mean several related things depending on context, including sky, heaven, or weather.

In Trời hôm nay nóng, it is understood as referring to the weather or the atmosphere outside, not just the physical sky.

That is why the sentence means It’s hot today rather than literally talking only about the sky.

This is very common in Vietnamese:

• Trời nóng. = It’s hot.
• Trời lạnh. = It’s cold.
• Trời mưa. = It’s raining.

Yes, Hôm nay trời nóng is also natural.

Vietnamese is often flexible about time expressions like hôm nay (today). You can place them in different positions depending on focus and style.

Common versions:

• Trời hôm nay nóng.
• Hôm nay trời nóng.

Both are natural. The difference is mainly emphasis:

• Hôm nay trời nóng puts today first, so it highlights the time.
• Trời hôm nay nóng starts with trời, so it feels more like the weather today is hot.

Both are perfectly usable in everyday speech.

Nóng is a general word meaning hot/warm, depending on context. It is not limited to weather.

Examples:

• Trời nóng. = The weather is hot.
• Nước nóng. = Hot water.
• Cà phê nóng. = Hot coffee.
• Tôi nóng quá. = I’m hot (I feel hot)

So in this sentence, nóng describes the weather, but the word itself is broader.

Yes. That is completely natural.

• Trời hôm nay nóng. = It’s hot today.
• Trời hôm nay rất nóng. = It’s very hot today.

Rất means very and goes before the adjective:

• rất nóng = very hot
• rất lạnh = very cold

This is a very common pattern in Vietnamese.

A learner will usually want to pay attention to both the sounds and the tones.

A rough English-friendly guide:

• trời ≈ chuh-ee / choy-ee with a falling-rising contour
• hôm ≈ home but shorter
• nay ≈ nye or nai depending on accent
• nóng ≈ nong with a rising tone

A few important pronunciation points:

• tr in southern speech often sounds closer to English ch.
• tr in northern speech is different from ch, though many learners hear them as similar at first.
• ời in trời is a vowel combination that takes practice.
• nóng has a tone, so it is not the same as a flat nong.

If you are learning tones, these are the standard northern tones:

• trời — huyền tone
• hôm — ngang tone
• nay — ngang tone
• nóng — sắc tone

It is neutral and very natural.

Trời hôm nay nóng is an everyday sentence that works in normal conversation. It is not especially formal or especially slangy.

You could hear this in speech, and it would also look fine in simple writing.

A few natural alternatives:

• Hôm nay trời nóng.
• Trời nóng quá. = It’s so hot.
• Hôm nay nóng ghê. = more expressive, casual

Yes, you can say Hôm nay nóng, and native speakers will understand it from context.

However, trời makes the sentence feel more complete and explicit when you are talking about the weather.

Compare:

• Hôm nay trời nóng. = clearly about the weather
• Hôm nay nóng. = still understandable, but more context-dependent

In conversation, dropping words is common when the meaning is obvious.

English weather sentences often use a dummy subject:

• It is hot today.

That it does not refer to a real thing in the same way as the book or the car.

Vietnamese usually does not need that kind of dummy subject. Instead, it often uses a meaningful word like trời or simply states the condition directly.

So instead of an empty it, Vietnamese says:

• Trời hôm nay nóng or
• Hôm nay nóng

This is a normal structural difference between English and Vietnamese.

No, that would sound unnatural.

The word là is not used here because this is a simple descriptive sentence with an adjective predicate.

Use:

• Hôm nay trời nóng.

Not:

• Hôm nay là trời nóng.

A simple way to remember it: If you are saying that something is hot/cold/beautiful/big, Vietnamese often just uses the adjective directly, without là.

Normally, no.

Trời hôm nay nóng is understood as a weather statement, meaning It’s hot today.

If you wanted to talk specifically about the sun, you would usually use mặt trời:

• Mặt trời hôm nay rất gắt = The sun is very harsh/strong today

So trời here is about the general outdoor conditions, not the sun itself.

This pattern is very common. You can swap nóng for other weather words:

• Trời hôm nay lạnh. = It’s cold today.
• Trời hôm nay mát. = It’s cool today.
• Trời hôm nay đẹp. = The weather is nice today.
• Trời hôm nay xấu. = The weather is bad today.
• Trời hôm nay âm u. = It’s gloomy today.

You can also move hôm nay:

• Hôm nay trời lạnh.
• Hôm nay trời đẹp.

This makes Trời hôm nay nóng a very useful model sentence.