Это было просто недоразумение, а не серьёзная ссора.

Because the subject это is treated as neuter singular, so the past-tense form of быть is было.

• он был = he was
• она была = she was
• оно было = it was
• это было = this/it was

Even though это can refer to many kinds of things in English, in Russian it usually takes neuter singular agreement in this kind of sentence.

In this sentence, это means something like it, this, or that situation.

Russian often uses это very broadly to point to a situation, event, or statement:

• Это было ошибкой. = It was a mistake.
• Это было странно. = That was strange.

So here это было просто недоразумение means It was just a misunderstanding.

In Russian, the verb быть (to be) is usually omitted in the present tense, but it appears in the past and future.

Compare:

• Это просто недоразумение. = It is just a misunderstanding.
• Это было просто недоразумение. = It was just a misunderstanding.

So in the past, you need было.

Here просто means just or simply.

It softens the statement and makes it sound like:

• just a misunderstanding
• nothing more than a misunderstanding

It can also mean simply in other contexts, but here just is the most natural translation.

Недоразумение is in the nominative singular.

In sentences like Это было X, the noun after это было is often in the nominative:

• Это была ошибка.
• Это было недоразумение.
• Это был хороший день.

So недоразумение is the basic dictionary form here.

Also, недоразумение is a neuter noun, which is why it ends in -е.

А не is the normal way to say rather than / and not / not X but Y in contrasts like this.

So:

• просто недоразумение, а не серьёзная ссора = just a misunderstanding, not a serious quarrel

Russian uses а when contrasting two ideas side by side.

Compare the feel:

• не X, а Y = not X, but Y
• X, а не Y = X, not Y

Using но here would sound less natural. Но usually means but in a more general sense, while а is especially common for direct contrast.

Because ссора is a feminine singular noun, so the adjective must agree with it.

• серьёзный = masculine
• серьёзная = feminine
• серьёзное = neuter
• серьёзные = plural

Since ссора is feminine, we get:

• серьёзная ссора = a serious quarrel

This is standard adjective-noun agreement in Russian.

Yes, ё shows the stressed sound yo.

So серьёзная is pronounced roughly ser-YOZ-na-ya.

In many printed texts, Russians often write е instead of ё, so you may see:

• серьезная

But it still means the same word. For learners, it is helpful to remember that the correct pronunciation includes ё.

The comma is used because а is a coordinating conjunction connecting two contrasted parts of the sentence.

The structure is:

• Это было просто недоразумение, а не серьёзная ссора.

Russian punctuation usually separates such contrasting clauses or phrase groups with a comma.

Yes, Russian word order is flexible, but the original order is the most natural and neutral.

Original:

• Это было просто недоразумение, а не серьёзная ссора.

You could also say:

• Это было не серьёзная ссора, а просто недоразумение.

That version puts stronger emphasis on correcting the wrong idea: It wasn’t a serious quarrel, it was just a misunderstanding.

So the word order changes the emphasis more than the basic meaning.

These words are related but not identical:

• ссора = quarrel, fight, falling-out
  Usually personal and emotional.
• спор = argument, dispute
  Often about opinions; can be less emotional.
• конфликт = conflict
  Broader and often more serious or formal.

So серьёзная ссора suggests a real personal quarrel, not just a small misunderstanding.

Yes, it is a common and useful word. It usually means misunderstanding in a fairly neutral tone.

In this sentence, it makes the situation sound less serious:

• not a real fight
• just a misunderstanding
• something caused by confusion, not hostility

So the whole sentence downplays the problem.