Я не только не сохранила резервную копию, но и забыла сделать скриншот.

Because two different things are happening at once:

• не только ... но и ... = not only ... but also ...
• the second не directly negates the verb сохранила = did not save

So the structure is:

• не только не сохранила резервную копию
• но и забыла сделать скриншот

In natural English, this is like:

• I not only didn’t save a backup, but also forgot to take a screenshot.

So the double не is normal here, not a mistake.

It is a fixed correlative structure meaning not only ... but also ....

Pattern:

• не только X, но и Y

In this sentence:

• X = не сохранила резервную копию
• Y = забыла сделать скриншот

It emphasizes that the speaker made two mistakes, not just one.

Without this structure, the sentence would still be understandable, but less emphatic:

• Я не сохранила резервную копию и забыла сделать скриншот.

That simply lists two actions.
Не только ... но и ... highlights the buildup: not only this, but also that.

Because they are past tense, feminine singular forms.

In Russian past tense, verbs agree with the subject in gender and number:

• сохранил, забыл = masculine singular
• сохранила, забыла = feminine singular
• сохранило, забыло = neuter singular
• сохранили, забыли = plural

So this sentence suggests that the speaker is female.

If a male speaker said the same thing, it would be:

• Я не только не сохранил резервную копию, но и забыл сделать скриншот.

Because резервную копию is in the accusative case, since it is the direct object of сохранила.

Dictionary form:

• резервная копия = backup copy

After the verb сохранить / сохранять when you say what was saved, Russian uses the direct object:

• сохранить что? → резервную копию

Since копия is a feminine noun, both the adjective and noun change in the accusative singular:

• резервная копия → nominative
• резервную копию → accusative

Сохранила comes from сохранить, the perfective verb.

Here, perfective is used because the speaker is talking about one specific completed action that should have happened but didn’t: saving the backup at that moment.

Compare:

• Я не сохранила резервную копию.
  = I didn’t save the backup.
  One concrete missed action.

• Я не сохраняла резервную копию.
  = I wasn’t saving a backup / I didn’t save backups
  This sounds more process-based, repeated, or context-dependent.

In this sentence, the idea is a single failure in a specific situation, so perfective is the natural choice.

Because Russian uses забыть + infinitive to mean to forget to do something.

So:

• забыла сделать скриншот = forgot to take a screenshot

But:

• забыла скриншот

would mean something more like forgot the screenshot itself, as an object, which does not fit this context.

This is the same difference as in English:

• I forgot to take a screenshot ✅
• I forgot a screenshot ❌ or at least unnatural in this meaning

So забыла сделать is the normal pattern here.

Because сделать is perfective, and it refers to one complete action: taking one screenshot.

• забыла сделать скриншот = forgot to take a screenshot

If you used делать, it would usually suggest an ongoing, repeated, or habitual action:

• забыла делать скриншоты = forgot to take screenshots as a habit / forgot that she was supposed to do that regularly

Here the speaker means one missed action in one situation, so сделать is the best choice.

Because after не только, Russian very often uses the matching pair но и.

So the full structure is:

• не только ..., но и ...

This is the standard equivalent of not only ..., but also ...

The и adds the sense of also / too.

Russian sometimes allows variation in real speech, but for learners, it is best to treat не только ... но и ... as a set pattern.

Yes, very often.

Russian frequently drops subject pronouns when they are clear from the verb form or context. So this would still be natural:

• Не только не сохранила резервную копию, но и забыла сделать скриншот.

Because сохранила and забыла already show:

• first person
• singular
• feminine

The pronoun Я is included here for clarity or emphasis, but it is not strictly necessary.

Yes.

In this sentence, the comma separates the two parts of the correlative construction:

• Я не только не сохранила резервную копию, но и забыла сделать скриншот.

Russian normally uses a comma before но in this kind of structure, just as English uses punctuation in not only ..., but also ... sentences.

So the comma here is standard and expected.

Yes, скриншот is very common in modern Russian, especially in everyday speech, tech contexts, and online communication.

You may also see:

• снимок экрана

which is more literally screen capture or screen image, and can sound a bit more formal or technical.

So:

• сделать скриншот = very common, natural
• сделать снимок экрана = also correct, often more formal

This sentence mixes a more standard/native-style phrase:

• резервная копия

with a common borrowing:

• скриншот

That is completely normal in modern Russian.