Это была случайная царапина.

Because была agrees with царапина, which is a feminine singular noun.

In Russian sentences like Это был/была/было ..., the past-tense form often matches the gender and number of the noun that follows:

• Это был врач. — masculine
• Это была ошибка. — feminine
• Это было письмо. — neuter

So since царапина is feminine, you get была.

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

In Russian, это is very commonly used in identification sentences:

• Это мой брат. — This is my brother.
• Это была ошибка. — That/It was a mistake.

So in Это была случайная царапина, это points to some mark, injury, or thing being discussed and identifies it as a(n) accidental scratch.

Because it describes царапина, and adjectives in Russian must agree with the noun they describe in gender, number, and case.

• царапина = feminine, singular, nominative
• so the adjective must also be feminine, singular, nominative: случайная

Compare:

• случайный порез — an accidental cut (masculine)
• случайная царапина — an accidental scratch (feminine)
• случайное ранение — an accidental wound (neuter)

It is in the nominative case.

In this kind of sentence, царапина is the noun that identifies what это was, and Russian normally uses the nominative in this pattern:

• Это была ошибка.
• Это был подарок.
• Это была случайная царапина.

So both случайная and царапина are nominative singular feminine.

Because with это in identification sentences, Russian usually uses the nominative, not the instrumental.

A learner may know sentences like:

• Он был врачом. — He was a doctor.

There the instrumental is common. But with это sentences, nominative is standard:

• Это был врач.
• Это была проблема.
• Это была случайная царапина.

So царапина in the instrumental, царапиной, would not be the normal form here.

Here it means accidental.

The adjective случайный can have several related meanings depending on context, including:

• accidental
• random
• chance
• casual

But with something like an injury or mark, случайная царапина is most naturally understood as an accidental scratch, not a random scratch.

Yes. In the present tense, Russian usually does not use a present-tense form of to be.

So:

• Это случайная царапина. — This is an accidental scratch.
• Это была случайная царапина. — This was an accidental scratch.

Russian normally leaves out есть in simple present-tense identification sentences.

You know from context, because Russian has no articles like a and the.

So случайная царапина could be understood in English as:

• an accidental scratch
• the accidental scratch

depending on the situation.

In many isolated examples, English will naturally use a/an, but Russian itself does not mark that difference directly.

The given order is the most neutral and natural one.

• Это была случайная царапина.

Russian word order is more flexible than English, but changing it usually changes the emphasis. For example, if you move words around, it may sound marked, contrastive, or context-dependent.

So for a learner, this is the best default order to use.

The stress is:

Э́то была́ случа́йная цара́пина.

A rough pronunciation guide:

• Э́то — EH-ta
• была́ — by-LA
• случа́йная — sloo-CHAI-na-ya
• цара́пина — tsa-RA-pi-na

Stress matters in Russian, so it is worth learning words together with their stress.

No. Царапина means a scratch, usually a small surface scratch or scrape, and it can be on:

• skin
• furniture
• a car
• glass
• other surfaces

So the noun is fairly broad, just like English scratch. In this sentence, context tells you whether it is a scratch on a person or on an object.

Because grammatical gender in Russian belongs to the noun, not to the real-world object in any biological sense.

Царапина is grammatically feminine because of its noun class and ending pattern, so related words must match that feminine gender:

• случайная царапина
• эта царапина
• царапина была

This has nothing to do with whether the scratch itself is somehow feminine in meaning.

You could translate it literally that way in some contexts, but in most normal situations accidental scratch is better.

For an English learner of Russian, the safest interpretation here is:

• It was an accidental scratch.
• That was just an accidental scratch.

Using random scratch in English would usually sound less natural unless you specifically mean something like a scratch that appeared for no obvious pattern or reason.