Chất này rất độc.

Vietnamese often does not use a verb like to be when an adjective directly describes something.

So:

• Chất này rất độc
• literally: substance this very poisonous
• natural English: This substance is very poisonous

This is a very common Vietnamese pattern:

• Nước này lạnh = This water is cold
• Cô ấy đẹp = She is beautiful

You only need a word like là in other kinds of sentences, not normally before an adjective here.

In Vietnamese, demonstratives like this and that usually come after the noun.

So:

• chất này = this substance
• người đó = that person
• cuốn sách này = this book

This is one of the biggest word-order differences from English.
English says this substance, but Vietnamese says substance this.

Here, chất means substance, material, or chemical substance, depending on context.

In this sentence, it most naturally means something like:

• this substance
• this material
• this chemical

Be careful: chất has other meanings in other contexts. For example, it can also refer to quality, essence, or matter/substance in a broader sense. But in Chất này rất độc, the meaning is clearly the physical-substance sense.

No. Độc can mean different things depending on context.

In this sentence, because it follows chất (substance), it means:

• poisonous
• toxic

But in other contexts, độc can also mean:

• unique
• strange / unusual
• vicious or severe, in some expressions

So context matters a lot.
In Chất này rất độc, the meaning toxic/poisonous is the correct one.

Rất means very and comes before the adjective it modifies.

So:

• độc = poisonous
• rất độc = very poisonous

This is the normal pattern:

• rất nóng = very hot
• rất tốt = very good
• rất nguy hiểm = very dangerous

So the structure here is:

• Chất này = this substance
• rất độc = very poisonous

Yes. Chất này độc is perfectly grammatical.

The difference is just intensity:

• Chất này độc = This substance is poisonous
• Chất này rất độc = This substance is very poisonous

So rất adds emphasis.

No, that sounds unnatural in standard Vietnamese.

When an adjective describes the noun directly, you normally do not use là.

Correct:

• Chất này rất độc

Not natural here:

• Chất này là rất độc

A good rule for learners is:
when the predicate is an adjective like độc, đẹp, cao, ngon, you usually do not add là.

They are quite different.

• Chất này rất độc = This substance is very poisonous
• Đây là chất độc = This is a poison / toxic substance

In the first sentence, độc is an adjective describing the substance.

In the second, chất độc is a noun phrase meaning poison or toxic substance.

So:

• chất này = this substance
• chất độc = poison / toxic substance

That difference is important.

They are related, but not always identical.

• độc usually means poisonous / toxic
• độc hại often means harmful or toxic, and can sound a bit broader

Examples:

• Chất này rất độc = This substance is very poisonous/toxic
• Khói này độc hại = This smoke is harmful/toxic

For chemicals and substances, both may be possible depending on context, but độc feels very direct and strong.

Yes. It is natural, clear, and standard.

It sounds especially appropriate in contexts like:

• science
• medicine
• warnings
• discussing chemicals or materials

In very casual speech, people might use other wording depending on context, but Chất này rất độc is completely normal Vietnamese.

No, not here.

Chất itself is already the noun, and it refers to a substance or material, so no classifier is needed in this sentence.

Vietnamese classifiers are common with countable objects, but with many mass nouns or abstract/material nouns, you often do not use one in a basic statement like this.

So:

• Chất này rất độc is correct.

If you were talking about a bottle, sample, or type of substance, then the sentence might use different nouns or classifiers.

A learner-friendly pronunciation guide would be something like:

• Chất — starts with ch, then a short vowel, ends abruptly with t
• này — similar to nye with a falling tone
• rất — the r depends on accent; ends with a clipped t
• độc — starts with đ like a plain d sound, ends with c/k

A few important points:

1. Final stops are unreleased In words like chất, rất, and độc, the final consonant is short and clipped. You do not strongly release it the way English often does.

2. Tones matter Each syllable has its own tone, and changing the tone can change the meaning.

3. đ is not the same as d In Vietnamese spelling, đ is a separate letter.

Exact pronunciation varies by region, especially for r, but the sentence is the same in writing.

Because the adjective here is not inside the noun phrase; it is the predicate of the sentence.

Compare these two ideas:

1. predicate adjective

   • Chất này rất độc
   • This substance is very poisonous

2. adjective inside a noun phrase

   • chất độc
   • poison / toxic substance

So in Chất này rất độc, the structure is:

• Chất này = topic/subject
• rất độc = comment/predicate

That is why it does not work like English adjective order.

Yes.

• Chất này rất độc = This substance is very poisonous
• Chất đó rất độc = That substance is very poisonous
• Chất kia rất độc = That substance over there is very poisonous

Very roughly:

• này = this (near the speaker)
• đó = that
• kia = that over there / yonder

The exact feel can vary by context and region, but this is the basic distinction.

Yes, and it sounds natural in speech.

Both mean roughly This substance is very poisonous, but the feeling is a little different:

• rất độc = straightforward, neutral, standard
• độc lắm = often more spoken, expressive, sometimes more emphatic

So:

• Chất này rất độc sounds a bit more neutral or informational.
• Chất này độc lắm can sound more conversational or more like a warning.

Not exactly. Vietnamese nouns do not work the same way as English countable/uncountable nouns.

Chất can refer to substance in a general or specific way, and context usually tells you how to understand it.

So Chất này can mean:

• this substance
• this material
• this chemical

You do not need an article like a or the, because Vietnamese does not use articles the way English does.

A useful breakdown is:

• Chất này = noun phrase (this substance)
• rất = degree adverb (very)
• độc = adjective (poisonous/toxic)

So the overall pattern is:

Noun + demonstrative + degree word + adjective

That gives:

Chất này rất độc
= This substance is very poisonous

This is a very common Vietnamese sentence pattern.