Не только не расшифровала, но и забыла комбинацию.

It is the standard Russian pattern for not only ..., but also ....

So in this sentence:

• не только не расшифровала
• но и забыла комбинацию

the speaker is linking two facts and saying the second one makes the situation even worse.

A very literal breakdown is:

• не только = not only
• но и = but also

This is a very common structure in Russian.

Because the two не do different jobs.

1. The first не is part of the fixed expression не только ... но и ... = not only ... but also ...
2. The second не negates the verb расшифровала = did not decipher / decode

So:

• не только не расшифровала = not only did she not decipher it

This double не is completely normal in Russian with this construction.

Compare:

• Она не только опоздала, но и забыла документы.
  She not only arrived late, but also forgot the documents.

• Она не только не позвонила, но и выключила телефон.
  She not only didn’t call, but also turned off her phone.

Because it is a past tense feminine singular form.

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

• расшифровал = he deciphered
• расшифровала = she deciphered
• расшифровало = it deciphered
• расшифровали = they deciphered

The same applies to забыла:

• забыл = he forgot
• забыла = she forgot

So the sentence tells you the subject is feminine, even though the pronoun is not stated.

Because Russian often omits subject pronouns when they are clear from context.

The verb endings already tell you a lot:

• расшифровала
• забыла

Both are feminine singular past forms, so the listener can understand that the subject is she or some feminine noun already mentioned earlier.

Russian does this much more freely than English. English usually needs she, but Russian often does not.

So this sentence is natural even without она.

Комбинацию is the accusative singular form of комбинация.

Why accusative? Because it is the direct object of забыла:

• забыла что? = forgot what?
• комбинацию

For feminine nouns ending in -ия, the accusative singular usually ends in -ию:

• комбинация → комбинацию
• лекция → лекцию
• станция → станцию

So the ending shows that комбинацию is the thing that was forgotten.

Расшифровала comes from расшифровать, which usually means:

• to decipher
• to decode
• to decrypt
• to work out the meaning of something coded or unclear

Depending on context, it could be about:

• a code
• an encrypted message
• shorthand
• some hidden or hard-to-understand information

So the verb suggests that something had to be figured out or decoded.

Also notice that Russian often leaves out an object if it is already understood from context. So after не расшифровала, the object is implied rather than stated.

Because Russian often omits words that are clear from context.

In English, we might be more likely to say something like:

• didn’t decipher it

But in Russian, if everyone already knows what it refers to, the object can simply be left unstated:

• не расшифровала

This sounds natural if the thing being deciphered has already been mentioned or is obvious from the situation.

So the sentence does not mean the verb is being used incorrectly. It just means the object is understood.

Both are perfective verbs:

• расшифровать → perfective
• забыть → perfective

Perfective verbs in Russian usually present an action as a completed whole or focus on the result.

Here that fits well:

• she failed to decipher it as a completed event
• she forgot the combination as a completed event/result

The sentence is about what happened, not about an ongoing process.

If you used imperfective forms, the meaning would shift. For example, it could sound more like process, repetition, or background description rather than a single finished outcome.

Because не только ..., но и ... links two coordinated parts, and Russian normally separates them with a comma.

So:

• Не только не расшифровала, но и забыла комбинацию.

The comma marks the break between the first coordinated part and the second.

This is standard Russian punctuation with this kind of paired conjunction.

In context, комбинация often means a combination in the sense of:

• a lock combination
• a safe combination
• a number sequence used to open something

In natural English, depending on context, you might also say code.

So Russian комбинация is not always the same as the everyday English word combination in every situation. Here it most likely refers to a memorized sequence or access combination.

Context decides the best English wording, but grammatically the Russian word is perfectly normal.

Yes. Russian word order is fairly flexible, though the basic meaning would stay the same.

For example, you could say:

• Не только не расшифровала, но и комбинацию забыла.

This puts slightly more emphasis on комбинацию.

The original:

• Не только не расшифровала, но и забыла комбинацию.

is neutral and natural.

So word order in Russian can shift for emphasis, rhythm, or style, but the case endings and verb forms keep the sentence clear.

Yes. That is exactly one of the main effects of не только ..., но и ....

The structure often suggests escalation:

• one bad thing happened
• and then, on top of that, another thing happened

So the sentence is not just listing two facts. It presents them as a kind of buildup:

1. she didn’t decipher it
2. she even forgot the combination

That makes the second part feel like an added complication or a stronger point.

Yes, Russian has other ways to express a similar idea, but не только ..., но и ... is the most direct equivalent of not only ..., but also ....

For example:

• Она не просто не расшифровала это, а ещё и забыла комбинацию.

This is a bit more conversational and emphatic.

But the sentence you have is more standard and structurally neat:

• Не только не расшифровала, но и забыла комбинацию.

So a learner should definitely recognize не только ..., но и ... as an important core pattern.