Trời mưa suốt ngày.

Yes. Trời can mean sky, heaven, or more generally the weather / the outdoors, depending on context.

In Trời mưa suốt ngày, trời works a bit like the English dummy subject it in It’s raining. Vietnamese often uses trời in weather expressions:

• Trời mưa. = It’s raining.
• Trời nắng. = It’s sunny.
• Trời lạnh. = It’s cold.

So although trời literally has meanings like sky or heaven, here it is best understood as part of a natural weather expression.

Here, mưa is functioning as a verb, meaning to rain.

Vietnamese words often do not change form the way English words do, so the same word can be used in different ways depending on context.

For example:

• mưa = rain / to rain
• Trời mưa. = It is raining.
• Cơn mưa = a rain shower / a bout of rain

In Trời mưa suốt ngày, the structure is basically:

• trời = the weather / it
• mưa = rain
• suốt ngày = all day

So: It rained / was raining all day or It rains all day, depending on context.

Vietnamese does not need a verb like to be in this kind of sentence.

English says:

• It is raining.
• It was raining.

Vietnamese simply says:

• Trời mưa.

The time is usually understood from context, or shown with time words if needed:

• Hôm nay trời mưa. = Today it’s raining / it rained today.
• Hôm qua trời mưa. = Yesterday it rained.
• Bây giờ trời mưa. = It’s raining now.

So Trời mưa suốt ngày does not itself mark tense the way English does.

By itself, it can mean different things depending on context, but the most natural interpretation is usually:

• It rained all day or
• It was raining all day

If you are describing a general habit, you would usually add more context, for example:

• Mùa này trời mưa suốt ngày. = In this season, it rains all day.

If you are talking about right now, you might say:

• Hôm nay trời mưa suốt ngày. = It’s been raining all day today.

So Vietnamese often leaves tense/aspect to context rather than marking it directly.

Suốt ngày means all day, throughout the day, or the whole day long.

• suốt = throughout / continuously through
• ngày = day

Together, they express duration over the entire day.

Examples:

• Tôi làm việc suốt ngày. = I work all day.
• Nó ngủ suốt ngày. = He/She sleeps all day.

In your sentence, Trời mưa suốt ngày means the rain continued through the whole day.

Both can often be translated as all day, and in many everyday situations they are very close.

• suốt ngày emphasizes the idea of lasting throughout the whole day
• cả ngày means the whole day more generally

So:

• Trời mưa suốt ngày = It rained throughout the day.
• Trời mưa cả ngày = It rained all day.

In many contexts, either one is natural. Suốt ngày can sometimes feel a little more like an unbroken stretch or continuous duration.

Also, in some contexts suốt ngày can have an annoyed tone, like all day long:

• Nó nói suốt ngày. = He talks all day long.

But in your sentence, it is simply descriptive.

Yes, you can, especially in casual speech or when the weather topic is already clear.

• Mưa suốt ngày. = Rained all day / It’s been raining all day.

However, Trời mưa suốt ngày sounds fuller and very natural as a complete sentence. Including trời makes the weather expression more explicit.

So both are possible, but:

• Trời mưa suốt ngày = slightly fuller, standard weather statement
• Mưa suốt ngày = more concise, conversational

Because in Vietnamese, time-duration expressions often come after the verb or predicate.

So the structure is:

• Trời
  • mưa
    • suốt ngày
• subject/topic + action/state + duration

This is very natural in Vietnamese.

Compare:

• Tôi học suốt đêm. = I studied all night.
• Cô ấy làm việc suốt tuần. = She worked all week.

Putting suốt ngày at the end is the normal word order here.

A rough guide:

• trời — sounds somewhat like chuhy / chuh-ee with a rising tone, though this is only approximate
• mưa — roughly mưa is hard to represent in English; something like moo-uh said smoothly
• suốt — roughly soo-uht with a sharp falling-glottal tone
• ngày — roughly nguy / ngai with a rising tone, depending on accent

The tones are important:

• trời = huyền tone
• mưa = ngang tone
• suốt = sắc tone
• ngày = huyền tone

A more useful learner tip: it is better to learn pronunciation from native audio than from English-style spelling, because several Vietnamese vowels and consonants do not match English sounds closely.

It is neutral and completely natural.

Trời mưa suốt ngày is an ordinary everyday sentence. You could say it in conversation, write it in a message, or use it in simple narration.

It is not especially formal and not slangy either.

Not in normal usage. Even though trời can literally relate to sky or heaven, Vietnamese speakers understand Trời mưa as a weather expression meaning It’s raining.

So you should not translate it word-for-word as The sky rains all day in normal English. That would sound unnatural. The natural meaning is simply:

• It rained all day
• It was raining all day
• It has been raining all day

depending on context.

You can add time words, place words, or aspect words. For example:

• Hôm nay trời mưa suốt ngày. = It rained all day today.
• Ở đây trời mưa suốt ngày. = It rains here all day / It has been raining here all day.
• Hôm qua trời mưa suốt ngày. = It rained all day yesterday.
• Trời đã mưa suốt ngày. = It rained all day / It has rained all day.
• Trời đang mưa. = It is raining right now.

This shows a useful pattern: the basic sentence is simple, and you can make it more precise by adding context words around it.