Не то чтобы эта ошибка была серьёзной, но её лучше исправить.

Не то чтобы is a very common Russian phrase used to soften what you are about to say. It often means something like:

• it’s not that...
• not exactly that...
• I wouldn’t say that...

So Не то чтобы эта ошибка была серьёзной, но... does not flatly deny that the error is serious. It softens the statement and introduces a contrast:

• It’s not that this mistake is serious, but...
• I wouldn’t say this mistake is serious, but...

This structure is often used when the speaker wants to sound less blunt.

This is a very common learner question.

In this sentence, была does not really indicate past time. It appears because of the construction with чтобы.

After чтобы, Russian normally uses a form that looks like the past tense:

• чтобы он пришёл
• чтобы она была
• чтобы они сделали

In many cases, this is better understood as part of a subjunctive-like / non-indicative construction, not a literal past tense.

So in Не то чтобы эта ошибка была серьёзной, the form была is just the normal grammatical form after чтобы. It does not mean the mistake used to be serious.

Because after быть in the past tense, Russian very often uses the instrumental case for the predicate noun or adjective.

So:

• ошибка была серьёзной
• он был уставшим
• она была врачом

Here:

• серьёзная = nominative feminine singular
• серьёзной = instrumental feminine singular

Using the instrumental after была is standard and natural.

A learner often expects nominative because English says the mistake was serious, but Russian commonly prefers instrumental in this kind of predicate.

Yes, you could, but it sounds a bit different.

There are two main possibilities:

• была серьёзной — full adjective in the instrumental; very common and neutral
• была серьёзна — short adjective; also correct, but often a bit more formal, literary, or stylistically marked

In everyday speech, была серьёзной is usually the safer and more natural choice.

So the sentence as given sounds very normal.

Because её is the direct object of исправить.

Compare:

• она = she / it (subject form)
• её = her / it (object form)

In the sentence:

• эта ошибка is the thing being talked about
• later, её refers back to ошибка
• исправить её = to correct it

So она лучше исправить would be impossible here, because она is a subject form, not an object form.

Because in Russian, pronouns agree with the grammatical gender of the noun, not with whether it is a person.

• ошибка is feminine
• so the pronoun referring to it is feminine too: она / её / ей / ею

That means Russian uses the same forms for feminine inanimate nouns and for feminine animate nouns:

• ошибка — её
• девушка — её

So here её simply means it, referring to the mistake.

This is an impersonal Russian pattern:

• лучше + infinitive

It means:

• it is better to...
• it would be better to...
• you’d better...

So:

• её лучше исправить = it’s better to correct it

Russian often leaves the subject unstated in this pattern. Depending on context, it can mean:

• you should correct it
• we should correct it
• someone should correct it

The idea is more important than the exact subject.

Because исправить is perfective, and here Russian is talking about a single completed action: getting the mistake corrected.

• исправить = to correct, to fix completely
• исправлять = to be correcting, to correct repeatedly / as a process

In её лучше исправить, the speaker means that the error should be fixed as a completed action.

If you used исправлять, it would sound more like focusing on the process or repeated activity, which is not the point here.

Но means but, and it introduces the contrast:

• Не то чтобы эта ошибка была серьёзной — softens the first idea
• но её лучше исправить — gives the practical conclusion

So the whole sentence has this shape:

• I’m not saying X is very serious, but still, it should be fixed.

This is a very natural Russian way to balance two ideas:

1. don’t overstate the problem
2. still recommend action

Because the sentence has two parts joined by но:

• Не то чтобы эта ошибка была серьёзной
• но её лучше исправить

Russian normally puts a comma before но.

Also, the first part is a subordinate-style construction with не то чтобы, so the pause before но is natural and required in writing.

It is best learned as a set expression.

Word-for-word, the pieces are:

• не = not
• то = that / that thing
• чтобы = so that / that

But if you try to build the meaning from the individual words, it may feel confusing. In real usage, не то чтобы functions as one unit meaning something like:

• it’s not that...
• not exactly...
• I wouldn’t say...

So for learners, the most useful approach is:

• recognize it as one phrase
• learn the tone: softening, qualifying, hedging

Yes, Russian word order is flexible, but the given version is very natural.

The sentence:

• Не то чтобы эта ошибка была серьёзной, но её лучше исправить.

sounds neutral and idiomatic.

You could move things around in some contexts, but that would usually change emphasis. For example, stressing лучше or её might be possible in speech, but the standard written version is the one you see here.

For a learner, this is a good model to copy as-is.

The tone is mild, careful, and practical.

The speaker is not being dramatic:

• they are downplaying the seriousness with не то чтобы
• but still recommending action with её лучше исправить

So it sounds like:

• This isn’t a huge problem, but it should still be fixed.

That kind of soft, balanced phrasing is very common in Russian, especially when giving feedback or criticism politely.