Не только не открыли сейф, но и сломали механизм.

Because they do two different jobs:

• не только = not only
• the second не negates the verb открыли = did not open

So the structure is literally:

• не только = not only
• не открыли сейф = did not open the safe

Putting it together: not only did they not open the safe...

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

This is a very common Russian correlative construction meaning:

not only ..., but also ...

In your sentence:

• Не только не открыли сейф = They not only didn’t open the safe
• но и сломали механизм = but also broke the mechanism

So the full pattern is:

• не только X, но и Y
• not only X, but also Y

It is used to add a second point that is often stronger, worse, or more surprising than the first.

Russian often omits the subject when it is clear from context or when the speaker does not want to name it directly.

Here the verbs are:

• открыли
• сломали

Both are past plural, so the implied subject is something like they.

Depending on context, it can mean:

• they
• some people
• someone / people in general (an unspecified group)

So the sentence does not need они unless the speaker wants extra emphasis or contrast.

Because the hidden subject is plural: they or some people.

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

• открыл / сломал = he did
• открыла / сломала = she did
• открыло / сломало = it did
• открыли / сломали = they did

So открыли and сломали tell you that the doers are plural, even though the pronoun itself is omitted.

Because the sentence talks about the result of completed actions.

• открыли = opened / managed to open
• сломали = broke

With negation, не открыли often means did not succeed in opening or did not open (as a completed result).

And сломали механизм means the mechanism ended up broken.

If you used the imperfective:

• не открывали сейф = were not opening / did not open it as an activity
• ломали механизм = were breaking / kept damaging the mechanism

That would sound more like process or repeated action, not the finished outcome. Here the speaker is focused on what happened in the end.

They are in the accusative case because they are direct objects of the verbs:

• открыли что? → сейф
• сломали что? → механизм

Both nouns are masculine inanimate singular, and for that type of noun the accusative looks the same as the nominative:

• nominative: сейф, механизм
• accusative: сейф, механизм

So even though the form does not change, the case here is accusative.

It can suggest both, depending on context.

With a perfective verb under negation, Russian often implies lack of successful result:

• не открыли сейф = they didn’t open the safe / they failed to open the safe

In this particular sentence, because it continues with но и сломали механизм, the idea is strongly that they tried and were unsuccessful, and on top of that they caused damage.

So in context, failed to open is often a very natural interpretation.

Because the standard paired construction is:

• не только ..., но и ...

The и adds the sense of also / as well.

Compare:

• не только X, но и Y = not only X, but also Y

If you said only но, it would sound incomplete or less idiomatic in this structure.

So но и is the normal matching second half of не только.

Yes, Russian word order is flexible, but changing it usually changes the emphasis, not the basic meaning.

Your sentence is a very natural neutral version:

• Не только не открыли сейф, но и сломали механизм.

Other possible variants include:

• Сейф не только не открыли, но и сломали механизм.
• Не только сейф не открыли, но и сломали механизм.
• Не только не открыли сейф, но и механизм сломали.

These versions may sound more marked or place emphasis on different parts, but the original sentence is the cleanest and most standard.

Not exactly instead of. It means:

• they did not open the safe
• and in addition, they broke the mechanism

So the logic is additive, not simply contrastive.

If Russian wanted a more direct instead / rather idea, it might use a different structure, for example with а in some contexts. But не только ..., но и ... clearly means not only X, but also Y.

Very likely, yes, but it is not stated explicitly in the grammar.

From real-world logic, сломали механизм here is naturally understood as broke the safe’s mechanism. Russian often leaves such relationships unstated when they are obvious from context.

If someone wanted to be fully explicit, they could say:

• сломали механизм сейфа = broke the mechanism of the safe
• сломали его механизм = broke its mechanism

But in your sentence, just механизм is enough because the safe has already been mentioned.

Yes, that is grammatically correct.

Adding они makes the subject explicit:

• Они не только не открыли сейф, но и сломали механизм.

Russian often leaves pronouns out when they are not needed, so the version without они is also perfectly natural.

Very roughly:

• without они = more neutral, concise
• with они = more explicit, sometimes more emphatic

The sentence suggests escalation:

1. They didn’t open the safe.
2. Worse than that, they broke the mechanism.

That is exactly why не только ..., но и ... fits so well here. The second part sounds like an added negative consequence.

So the overall feeling is something like:

Not only did they fail to open the safe, they even broke the mechanism.