Это пари было ошибкой.

Because the verb in the past tense agrees with the noun пари.

• был = masculine
• была = feminine
• было = neuter
• были = plural

The noun пари is treated as neuter singular in Russian, so the correct form is было.

Пари is a neuter noun.

It is also an indeclinable noun, meaning its form does not change across cases. This is common with some borrowed words in Russian.

So you get:

• это пари
• нет пари
• к пари

The form пари stays the same, even though its grammatical role changes.

Because after быть in the past tense, Russian often uses the instrumental case for the noun that describes what something was.

So:

• ошибка = nominative, dictionary form
• ошибкой = instrumental singular

In this sentence, ошибкой means a mistake in the sense of was a mistake.

This pattern is very common:

• Он был врачом. = He was a doctor.
• Это было проблемой. = It was a problem.
• План оказался ошибкой. = The plan turned out to be a mistake.

Using the instrumental here sounds natural and standard.

Normally, no. In standard Russian, ошибкой is the expected form here.

With быть in the past or future, the predicate noun is very often in the instrumental:

• Это было шуткой.
• Он был учителем.
• Идея была ошибкой.

You may sometimes see nominative in certain special styles or constructions, but for a learner, было ошибкой is the correct and natural pattern to follow.

Here это means this.

So Это пари = This bet

Russian это can be used in a few related ways:

• as this/that before a noun: это пари
• as it/this/that in a sentence like Это важно. = This/It is important.

In your sentence, это points to a specific bet.

Yes.

• Это пари было ошибкой. = This bet was a mistake.
• Пари было ошибкой. = The bet was a mistake / Betting was a mistake, depending on context

Adding это makes it more specific: this particular bet.

Without это, the sentence can sound a bit more general or depend more on context.

Russian does not have articles like a or the.

So ошибкой can mean:

• a mistake
• the mistake

The exact meaning depends on context.

That is why ошибкой by itself is enough.

No, Russian word order is fairly flexible, though some orders sound more neutral than others.

The neutral version here is:

• Это пари было ошибкой.

You could also change the order for emphasis, for example:

• Ошибкой было это пари.
  This emphasizes mistake.

• Было ошибкой это пари.
  Less neutral, more stylistic.

For learners, the original order is the safest and most natural.

Yes, but Russian handles the present tense of to be differently.

Russian usually omits the present-tense verb to be, so instead of saying a word for is, you normally just say:

• Это пари — ошибка.

This means This bet is a mistake.

The dash often helps show the link between the subject and the predicate in writing.

Compare:

• Это пари было ошибкой. = This bet was a mistake.
• Это пари — ошибка. = This bet is a mistake.

A natural pronunciation is:

Э́то пари́ бы́ло оши́бкой.

Stress:

• э́то
• пари́
• бы́ло
• оши́бкой

A rough pronunciation guide:

• это ≈ EH-tuh
• пари ≈ puh-REE
• было ≈ BY-luh
• ошибкой ≈ uh-SHEEP-kuy

The most important thing is to remember the stress in пари́ and оши́бкой.

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

The structure is:

• Это пари = subject
• было = verb
• ошибкой = predicate noun in the instrumental

So grammatically, the sentence works like:

[Subject] + [was] + [what it was]

which is why you get:

• nominative for the subject
• instrumental for ошибкой