Это серьёзное нарушение правил.

Because it agrees with нарушение, which is a neuter singular noun.

• нарушение = a violation / breach
• It is neuter, so the adjective must also be neuter:
  • masculine: серьёзный
  • feminine: серьёзная
  • neuter: серьёзное

So серьёзное нарушение means a serious violation.

In Russian, many nouns ending in -ение are neuter.
So нарушение is neuter singular.

That is why the adjective is серьёзное, not серьёзный or серьёзная.

Because after нарушение Russian normally uses the genitive case to show what was violated.

• правила = rules (nominative/accusative plural)
• правил = of rules (genitive plural)

So:

• нарушение правил = violation of rules

This is a very common Russian pattern: a noun is followed by another noun in the genitive to express something like of ....

In the present tense, Russian usually omits the verb to be.

So English This is a serious violation of the rules becomes simply:

• Это серьёзное нарушение правил.

There is no present-tense word corresponding to is here.

Here это means something like this, that, or it depending on context.

In sentences like this, это is very common when identifying or characterizing something:

• Это проблема. = This is a problem.
• Это ошибка. = This is a mistake.
• Это серьёзное нарушение правил. = This is a serious violation of the rules.

So это introduces what something is.

Not quite.

Russian has:

1. Demonstrative adjective forms that agree with a noun:

   • этот стол = this table
   • эта книга = this book
   • это письмо = this letter
   • эти книги = these books

2. Это used on its own as this / it / that is ...

   • Это важно. = This is important.
   • Это серьёзное нарушение правил.

In your sentence, это is being used on its own, not directly modifying a following noun.

It is in the nominative singular.

In this kind of sentence, the thing being identified is normally in the nominative:

• Это проблема.
• Это ошибка.
• Это серьёзное нарушение правил.

So нарушение is nominative singular, and серьёзное matches it.

Yes, Russian word order is fairly flexible, but the neutral/basic order here is:

• Это серьёзное нарушение правил.

You may also hear:

• Это нарушение правил серьёзное.

That version puts more emphasis on серьёзное, almost like This violation of the rules is serious.

So the original sentence is the most straightforward, neutral way to say it.

It is a very common and natural collocation.

Russian often uses a noun plus genitive structure like this:

• нарушение правил = violation of rules
• соблюдение правил = observance of rules
• изучение языка = study of a language
• обсуждение проблемы = discussion of a problem

So нарушение правил is not completely fixed in the sense that it is the only possible wording, but it is a standard and very common way to express the idea.

Yes, but it means something different.

• нарушение правил = violation of rules / rule violation in general, or of multiple rules
• нарушение правила = violation of a rule / of one specific rule

So your sentence with правил is broader and more natural in many contexts, especially when speaking generally about rules or regulations.

The ё is important because it shows the stressed yo sound.

• серьёзное is pronounced roughly sir-YOZ-na-ye

If you replace ё with е in writing, many native speakers will still understand it, because Russian texts often omit the dots over ё. So you may often see:

• серьезное

But it is still pronounced with yo, not plain e.

Sometimes, depending on context. Literally and most safely, нарушение means:

• violation
• breach
• sometimes breaking

For example:

• нарушение закона = violation/breach of the law
• нарушение правил = violation/breach of the rules

In English, breaking the rules sounds more natural in many everyday situations, but grammatically the Russian noun нарушение is a noun meaning violation or breach.