Он не заметил опасности.

In Russian, standard negation of a verb is formed with не placed before the verb:

• Он не заметил опасности. = He did not notice the danger.

If you say не before the noun instead, you are usually negating or contrasting the noun itself, as in:

• Он заметил не опасность, а что-то другое.
  He noticed not danger, but something else.

So in your sentence, не belongs with the verb заметил.

Заметил is the past tense, masculine singular form of the verb заметить.

Breakdown:

• заметить = to notice
• заметил = noticed / did notice (masculine subject)
• заметила = noticed (feminine subject)
• заметило = noticed (neuter subject)
• заметили = noticed (plural)

Because the subject is он (he), the masculine form заметил is used.

This is one of the most common questions learners ask.

Here, опасности is the genitive singular of опасность (danger).

After a negated verb, Russian often uses the genitive instead of the accusative, especially when the object is:

• abstract,
• indefinite,
• or viewed in a general sense.

So:

• Он не заметил опасности = He didn’t notice any danger / the danger

This is very natural Russian.

You may also hear accusative after negation in modern Russian in some contexts, especially with more definite, concrete objects, but with опасность in this sentence, опасности sounds especially normal and idiomatic.

Yes. This sentence is a classic example of what learners often call the genitive of negation.

With some negated transitive verbs, Russian can put the direct object in the genitive instead of the accusative:

• Я не вижу дома. = I don’t see the house / home
• Он не заметил опасности. = He didn’t notice the danger

This is not automatic in every sentence, but it is very common, and abstract nouns like опасность often appear this way after negation.

Russian has no articles like a, an, or the.

So опасность can mean:

• danger
• a danger
• the danger

The exact meaning depends on context.

In Он не заметил опасности, English often translates it as He didn’t notice the danger or He didn’t notice any danger, depending on the situation.

Good question. Заметил tells us only that the subject is masculine singular in the past tense. It does not tell us person by itself.

So заметил could mean:

• I noticed (if the speaker is male)
• you noticed (addressing a man, in context)
• he noticed

That is why он is useful: it clearly tells you the subject is he.

In context, Russian often drops pronouns when they are already understood, so you may also see:

• Не заметил опасности.

Depending on context, that could mean He didn’t notice the danger, or I didn’t notice the danger, if the speaker is male.

This is about aspect.

• заметить = perfective
• замечать = imperfective

The perfective verb заметить is used when talking about a completed event: the person either noticed something or failed to notice it at a particular moment or in a particular situation.

So:

• Он не заметил опасности.
  He didn’t notice the danger.
  → a completed fact

Compare:

• Он не замечал опасности.
  He wasn’t noticing the danger / He failed to notice the danger over some period
  → process, repeated situation, or ongoing lack of awareness

In your sentence, Russian presents this as a single completed event, so заметил is the natural choice.

The dictionary form is опасность.

Its basic forms are:

• nominative: опасность
• genitive: опасности
• dative: опасности
• accusative: опасность
• instrumental: опасностью
• prepositional: опасности

In your sentence, the form is genitive singular.

The stress is:

• Он
• не заме́тил опа́сности

More specifically:

• заме́тил → stress on ме
• опа́сности → stress on па

So you can pronounce it as:

он не замЕтил опАсности

Yes. Russian word order is flexible, though the neutral order here is:

• Он не заметил опасности.

Other orders are possible, for example:

• Опасности он не заметил.
  This puts more focus on danger.

• Не заметил он опасности.
  This can sound more marked, emotional, or stylistic depending on context.

For a learner, Он не заметил опасности is the safest and most neutral version.

Yes. Because Russian has no articles, and because the object is in the genitive after negation, the sentence can easily suggest:

• He didn’t notice any danger
• He didn’t notice the danger

Context decides which English translation is best.

If the situation is specific and everyone already knows what danger is meant, English may prefer the danger.
If the meaning is more general, English may prefer any danger.