Hôm nay trời mát mẻ.

• Hôm nay = today
• trời = sky, but very often it means the weather
• mát mẻ = cool, pleasantly cool, fresh

So the structure is basically:

Today + weather + cool/pleasant

A very natural English rendering is The weather is cool today or Today the weather is nice and cool.

Vietnamese often does not use a verb like English to be when describing something with an adjective.

So in English:

• The weather is cool

But in Vietnamese:

• Trời mát mẻ

There is no need to add a separate word for is here. This is completely normal Vietnamese sentence structure.

Yes, trời literally can mean sky or heaven, but in everyday Vietnamese it is also very commonly used when talking about weather.

For example:

• Trời nóng. = The weather is hot.
• Trời lạnh. = The weather is cold.
• Trời đẹp. = The weather is nice.

So in Hôm nay trời mát mẻ, trời is best understood as the weather.

Both relate to coolness, but mát mẻ usually sounds more descriptive and pleasant.

• mát = cool
• mát mẻ = pleasantly cool, fresh, nice and cool

So:

• Trời mát = The weather is cool.
• Trời mát mẻ = The weather is pleasantly cool.

The doubled form often gives a softer, more natural descriptive feeling.

Yes. This is a very normal Vietnamese pattern:

time + subject/topic + description

So:

• Hôm nay = time expression
• trời = topic/subject
• mát mẻ = description

Vietnamese often puts time expressions near the beginning of the sentence, especially in simple everyday statements.

Yes, Trời hôm nay mát mẻ is also grammatical and natural.

The difference is mainly in emphasis:

• Hôm nay trời mát mẻ emphasizes today
• Trời hôm nay mát mẻ starts with the weather

In everyday conversation, Hôm nay trời mát mẻ is probably the more common version.

Yes. Trời mát mẻ means The weather is pleasantly cool.

You add hôm nay only when you want to specify today. If the time is already clear from context, Vietnamese often leaves it out.

No. Mát mẻ can describe anything that feels pleasantly cool or fresh, depending on context.

For example, it can describe:

• weather
• air
• a room
• a place under shade

But in Hôm nay trời mát mẻ, because of trời, it clearly refers to the weather.

A rough pronunciation guide is:

• Hôm — mid/falling-ish in the northern standard, with the ô vowel
• nay — level
• trời — rising broken tone
• mát — high rising/sharp tone
• mẻ — dipping/rising broken tone

The hardest parts for many English speakers are usually:

• tr in trời
• the vowel in trời
• the tones on trời, mát, and mẻ

It is best to learn this by listening to native audio, because Vietnamese tones are very important for sounding natural and being understood.

It is neutral and very natural. You can use it in normal conversation without any problem.

It is not slang, and it is not especially formal either. It is just a standard everyday sentence.

Here it means cool in temperature, not cool/stylish.

Because the sentence is about trời = weather, mát mẻ can only be understood as pleasantly cool. It does not have the English slang meaning of cool.

It is possible in some contexts, but it is less common for this kind of simple statement.

The most natural choices are:

• Hôm nay trời mát mẻ.
• Trời hôm nay mát mẻ.

Putting the time expression first is very common in Vietnamese, so learners should feel comfortable using that pattern.