Не хмурься: это была всего лишь маленькая ошибка.

Basically, yes. Не хмурься means don’t frown, don’t look so gloomy, or don’t be upset.

The verb хмуриться is about looking dark, gloomy, or displeased, especially in your face. So in context, it is often a comforting line rather than a strict command. It can sound like:

• Don’t frown
• Don’t look so upset
• Don’t be glum

So the sentence has a reassuring tone.

Because the base verb is хмуриться, not хмурить.

The -ся ending often makes a verb reflexive or intransitive. Here it gives the meaning to frown / to look gloomy oneself.

Compare:

• хмурить брови = to knit/furrow one’s brows
• хмуриться = to frown / to look gloomy

So не хмурься is the natural way to say don’t frown when speaking to one person informally.

Yes. Хмурься is the imperative form: frown / be gloomy. With не, it becomes don’t frown.

So:

• хмурься = frown
• не хмурься = don’t frown

This is the singular informal command, used when speaking to one person as ты.

If you wanted the polite or plural version, you would say:

• не хмурьтесь

Russian usually leaves out ты and вы in commands unless you want extra emphasis.

Because in sentences like this, the past-tense form of быть often agrees with the noun that identifies what это is.

Here the key noun is ошибка, which is feminine, so you get:

• это была ошибка

Compare:

• Это был фильм — film is masculine
• Это было письмо — letter is neuter
• Это была ошибка — mistake is feminine

So even though это does not change, the past-tense verb changes to match the noun.

Because это была ошибка is an identifying statement: it was a mistake.

With это in this kind of sentence, the nominative is very common and natural:

• Это была ошибка

Russian can also use the instrumental:

• Это было ошибкой

But that version has a slightly different feel: more like that was a mistake as a judgment or evaluation.

So:

• Это была ошибка = identifying what it was
• Это было ошибкой = evaluating it as a mistake

In your sentence, ошибка in the nominative is the most straightforward choice.

Всего лишь is a common fixed expression meaning:

• only
• just
• merely

So это была всего лишь маленькая ошибка means it was only/just a small mistake.

The two words together add emphasis: nothing more than.

You can compare:

• лишь маленькая ошибка = only a small mistake
• всего лишь маленькая ошибка = just/only a small mistake, with stronger reassurance

So всего лишь is very natural here because the speaker is trying to minimize the seriousness of the mistake.

It is completely natural in Russian.

Russian often uses маленький not only for physical size, but also for things that are minor in importance. So маленькая ошибка means:

• a small mistake
• a minor mistake

In English, minor mistake may sound slightly more natural in some contexts, but Russian маленькая ошибка is perfectly normal, especially in speech.

A more neutral or formal alternative would be:

• небольшая ошибка

So the difference is roughly:

• маленькая ошибка = more everyday, warm, conversational
• небольшая ошибка = more neutral or formal

In this sentence, это works like it / this / that, depending on context.

Russian это is very common in sentences that identify or comment on something:

• Это ошибка = It’s a mistake / This is a mistake
• Это была ошибка = It was a mistake / That was a mistake

So here это refers to the thing that happened — the mistake the speaker is talking about.

A useful point: это itself does not show gender here. The gender shows up in была, which agrees with ошибка.

The colon introduces the explanation or reason for the first part.

So the structure is:

• Don’t frown: it was only a small mistake.

In other words:

• Don’t frown, because it was only a small mistake.

A dash could also be possible in some contexts, but the colon neatly marks the second clause as an explanation of the first.

Because всего лишь is limiting the whole noun phrase маленькая ошибка.

It means:

• only a small mistake
• just a small mistake

So the normal order is:

• это была всего лишь маленькая ошибка

That is the most natural placement. If you move всего лишь, the sentence may sound more marked or less natural.

Russian word order is flexible, but particles like всего лишь usually go before the word or phrase they modify. Here they modify the entire idea a small mistake.

Yes — the meaning stays the same, but the addressee changes.

• не хмурься = speaking to one person informally
• не хмурьтесь = speaking to one person politely or to several people

So if you were talking to a friend, child, sibling, or someone you address with ты, не хмурься is right.

If you were speaking formally, respectfully, or to a group, you would use:

• Не хмурьтесь: это была всего лишь маленькая ошибка.