Они заметили нарушение.

• Они = they
• заметили = noticed / spotted
• нарушение = violation / breach / infringement

So the sentence literally matches They noticed a violation or They noticed the violation, depending on context.

Заметить is the dictionary form, the infinitive: to notice.

Заметили is the past tense form used with они:

• я заметил / заметила = I noticed
• ты заметил / заметила = you noticed
• он заметил = he noticed
• она заметила = she noticed
• мы заметили = we noticed
• вы заметили = you noticed
• они заметили = they noticed

So заметили is the correct finite verb form for they noticed.

In Russian past tense, plural verbs usually end in -ли.

Here:

• infinitive: заметить
• past plural: заметили

The ending -ли tells you the subject is plural. Since они means they, заметили is exactly what you would expect.

Заметили comes from the perfective verb заметить.

Perfective verbs in Russian usually describe:

• a completed action,
• a single event,
• a result.

So Они заметили нарушение means they did notice it, as a completed event.

The imperfective partner is замечать:

• Они замечали нарушение would suggest repeated noticing or noticing as an ongoing/habitual process, which is much less natural in this context.

So here perfective is the normal choice because the sentence describes one completed act of noticing.

Нарушение is the direct object of the verb заметили, so it is in the accusative case.

But for many inanimate neuter nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: нарушение
• accusative: нарушение

That is why the word does not visibly change here.

Нарушение is neuter.

A common clue is the ending -ие. Many nouns ending in -ие are neuter, for example:

• здание = building
• решение = decision
• нарушение = violation

Its basic pattern is:

• nominative singular: нарушение
• genitive singular: нарушения
• dative singular: нарушению
• instrumental singular: нарушением
• prepositional singular: о нарушении

Yes, but Russian does not have articles like a and the.

So Они заметили нарушение can mean:

• They noticed a violation
• They noticed the violation

The exact meaning depends on context.

If this is new information, English often prefers a violation. If both speaker and listener already know which one is meant, English often prefers the violation.

Russian leaves that distinction to context instead of using articles.

Yes, often it can.

Russian frequently drops subject pronouns when the verb form already makes the meaning clear. So:

• Они заметили нарушение.
• Заметили нарушение.

Both can work, depending on context.

However, они may be included:

• for emphasis,
• for contrast,
• or when you want to make the subject especially clear.

No, Russian word order is relatively flexible because case endings show grammatical roles.

The neutral order here is:

• Они заметили нарушение.

But other orders are possible, for example:

• Нарушение они заметили. = It was the violation that they noticed.
• Заметили они нарушение. = more marked, depending on context or emphasis.

So the original sentence is the most straightforward, neutral version.

They are similar, but not identical.

• заметили = noticed, became aware of, spotted
• увидели = saw

Заметить often emphasizes that something was detected or recognized, sometimes after not being obvious at first.

For example:

• Они увидели нарушение = they saw the violation
• Они заметили нарушение = they noticed the violation

In many contexts, both are possible, but заметили often sounds a bit more like they observed / picked up on it.

A simple pronunciation guide is:

• Они ≈ a-NEE
• заметили ≈ za-MYE-ti-li
• нарушение ≈ na-roo-SHE-nye

The main stresses are:

• они́
• заме́тили
• наруше́ние

So the whole sentence sounds roughly like:

a-NEE za-MYE-ti-li na-roo-SHE-nye

Они is in the nominative case, because it is the subject of the sentence.

Its basic case forms are:

• nominative: они = they
• genitive/accusative: их
• dative: им
• instrumental: ими
• prepositional: о них

In this sentence, они is doing the action, so nominative is required.

Yes. Нарушение is a broad word and can refer to many kinds of violations, for example:

• a rule violation
• a law violation
• a safety breach
• a procedural breach
• a traffic violation
• a disciplinary infringement

The exact sense depends entirely on context. Russian uses this one noun quite broadly, much like English uses violation or breach in different settings.