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:
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:
In this sentence, ошибкой means a mistake in the sense of was a mistake.
This pattern is very common:
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:
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:
In your sentence, это points to a specific bet.
Yes.
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:
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:
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:
This means This bet is a mistake.
The dash often helps show the link between the subject and the predicate in writing.
Compare:
A natural pronunciation is:
Э́то пари́ бы́ло оши́бкой.
Stress:
A rough pronunciation guide:
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:
So grammatically, the sentence works like:
[Subject] + [was] + [what it was]
which is why you get: