Nước lũ đã vào thành phố.

Nước lũ means floodwater.

• nước = water
• lũ = flood

Together, nước lũ focuses on the actual water from a flood, not just the flood as an event.

So:

• lũ can mean a flood / flooding
• nước lũ means floodwater

In this sentence, nước lũ is natural because the thing physically moving into the city is the water.

Here it means water.

Vietnamese nước can mean both:

• water
• country / nation

The surrounding word tells you which meaning is intended. In nước lũ, it clearly means water, because lũ is related to flooding.

đã marks the action as having happened already or being completed/realized.

In many cases, it corresponds to English ideas like:

• already
• did
• has/have

So đã vào suggests that the floodwater has entered the city already.

A useful thing to remember: đã is not just a simple past tense marker in the same way English uses past tense. It is more about the event being realized or completed.

Here vào means enter, go into, or move into.

It is a very common Vietnamese verb of direction. Depending on context, it can be translated as:

• go in
• come in
• enter
• move into

With something like floodwater, English often sounds more natural with enter or reach and move into.

Because vào emphasizes movement into a place, while đến emphasizes arriving at a place.

Compare the nuance:

• vào thành phố = go/come into the city
• đến thành phố = arrive at the city

In this sentence, vào suggests that the water has actually crossed into the city area, not just reached its edge.

Because vào already contains the meaning of into.

Vietnamese often uses a directional verb followed directly by the destination:

• vào thành phố = enter the city
• ra đường = go out to the street
• lên núi = go up the mountain

So you do not need a separate preposition like English into here.

The structure is:

subject + aspect marker + verb + place

So in this sentence:

• Nước lũ = subject
• đã = aspect marker
• vào = verb
• thành phố = destination/place

That gives:

Nước lũ đã vào thành phố.

This is a very common Vietnamese pattern.

Vietnamese usually does not require articles like a, an, or the, and it often does not mark singular vs. plural unless needed.

So:

• thành phố can mean city or the city
• nước lũ can be understood as floodwater or the floodwaters, depending on context

English forces you to choose more grammar here than Vietnamese does.

Yes, it can be omitted if the context already makes the time clear.

For example:

• Nước lũ vào thành phố.

This could still be understood, especially in narration, headlines, or when the situation is obvious.

But adding đã makes it clearer that the event has already happened. It gives a stronger sense of the floodwater has already entered the city.

Yes, you could, but it is a bit more explicit.

• vào thành phố = enter the city
• vào trong thành phố = go into the inside of the city

Often, trong is unnecessary because vào already gives the basic idea of entering. Adding trong can make the sense of being inside more noticeable, but vào thành phố is already natural and complete.

Sometimes, yes, but the structure changes.

You may hear:

• Nước lũ vào thành phố rồi.

This also means the floodwater has already entered the city.

Very roughly:

• đã comes before the verb
• rồi usually comes after the verb phrase

Both can mark a completed or already-happened event, though their usage is not identical in every context. In many everyday sentences, either one can work. You may even see both together for emphasis:

• Nước lũ đã vào thành phố rồi.

A few words here can be hard for English speakers:

• nước: starts with n, then a vowel like ươ, ending with c; it has a rising tone
• lũ: has the ngã tone in standard Northern pronunciation, which can be tricky because it is not like an English tone
• đã: also has the ngã tone
• vào: lower/falling tone on và, then neutral o
• thành phố: thành has a low/falling tone; phố has a rising tone

If tones are still new to you, the most important thing is to learn the sentence in chunks:

• nước lũ
• đã vào
• thành phố

That often helps more than trying to perfect each word in isolation right away.