doku ha kiken desu.

It is read:

どく は きけん です
Romanized: doku wa kiken desu

A natural pronunciation is close to:

doku wa kiken des(u)

The u in desu is often very weak or almost silent in normal speech.

Here is the breakdown:

• 毒（どく） = poison
• は = topic marker
• 危険（きけん） = dangerous / danger
• です = polite sentence ending, similar to is/it is in this kind of sentence

So the sentence structure is basically:

As for poison, it is dangerous.

Natural English: Poison is dangerous.

This is one of the most common beginner questions.

The character は is normally read ha when it is part of a word.
But when it is used as the topic particle, it is pronounced wa.

So in this sentence:

• written: 毒 は 危険 です
• said: どく わ きけん です

This is just a standard rule of Japanese grammar.

は marks the topic of the sentence.

So 毒は means:

• as for poison
• speaking of poison

Then the rest of the sentence says something about that topic:

• 危険です = is dangerous

So Japanese is not literally word-for-word like English.
It is more like:

• As for poison, it is dangerous.

This is why は is often called the topic marker.

Japanese handles this differently from English.

In this sentence, です is the polite copula-like ending. It helps make the sentence polite and complete.

So:

• 危険です = is dangerous / is dangerous (polite)

In casual speech, you might hear:

• 毒は危険だ
• or even just 毒は危険

depending on context and level of formality.

So です often plays the role that is/am/are plays in English, but it does not behave exactly the same way.

Yes, but more specifically, 危険 is usually treated as a na-adjective (also called an adjectival noun).

That means:

• when it comes before a noun, it usually takes な
• when it ends the sentence, it can be followed by です or だ

Examples:

• 危険な物 = a dangerous thing
• この場所は危険です = This place is dangerous

So in 毒は危険です, 危険 is being used at the end of the sentence as the description.

Because な is only used when a na-adjective comes directly before a noun.

For example:

• 危険な食べ物 = dangerous food

But in your sentence, 危険 is not modifying a noun. It is the main predicate of the sentence:

• 毒は危険です = Poison is dangerous

So な is not needed.

Japanese usually does not mark articles like a or the, and it often does not mark plural unless it needs to.

So 毒 can mean:

• poison
• the poison
• poisons

The exact meaning depends on context.

In a general statement like this, 毒は危険です is naturally understood as:

• Poison is dangerous

That is, poison in general.

Sometimes, but the nuance changes.

• 毒は危険です = As for poison, it is dangerous.
  This sounds like a general statement about poison.

• 毒が危険です = Poison is the dangerous thing / It’s poison that is dangerous.
  This can sound more contrastive or like it answers a specific question, such as Which one is dangerous?

So for a basic general statement, は is the more natural choice.

Yes. It is grammatically correct and natural.

It sounds like a general factual statement, such as something from:

• a textbook
• a warning explanation
• a simple science statement

Depending on context, Japanese speakers might also say things like:

• 毒は危ないです = Poison is dangerous
• 毒は危険なものです = Poison is a dangerous thing

But 毒は危険です is perfectly fine.

In casual speech, yes, sometimes.

Polite form:

• 毒は危険です

Plain form:

• 毒は危険だ

Very casual or note-like style:

• 毒は危険

However, for learners, it is best to use です in polite sentences unless you are specifically studying casual speech.

Here are the common forms:

• 毒は危険です。 = Poison is dangerous.
• 毒は危険ではありません。 = Poison is not dangerous.
• 毒は危険でした。 = Poison was dangerous.
• 毒は危険ではありませんでした。 = Poison was not dangerous.

This is useful because many na-adjectives work this same way with です.

Normally, Japanese is written without spaces:

毒は危険です。

Spaces are often added in beginner materials to make the parts easier to see:

• 毒
• は
• 危険
• です

So the spaced version is just for learning convenience, not standard writing.