Hvis min nøgle bliver væk, bliver jeg nervøs.

Hvis means if and introduces a condition.

So Hvis min nøgle bliver væk, bliver jeg nervøs has the structure:

• Hvis ... = If ...
• main clause = what happens in that situation

It is a very common way to make conditional sentences in Danish.

It is the same verb, blive, but it works a little differently in each part.

• min nøgle bliver væk = my key gets lost / goes missing
• jeg bliver nervøs = I get nervous / become nervous

The basic idea of blive is become / get.

So in both cases there is a change of state:

• the key goes from not missing to missing
• I go from not nervous to nervous

This is very natural Danish.

Danish can express this idea in more than one way.

• Hvis min nøgle bliver væk focuses on the key: If my key goes missing / gets lost
• Hvis jeg mister min nøgle focuses on the person: If I lose my key

Both are possible, but they are not exactly the same in emphasis.

• bliver væk = the key ends up missing
• mister = I lose it

So the given sentence is perfectly natural, especially when the important thing is that the key is now gone.

Væk usually means away, gone, or missing, depending on context.

In bliver væk, it forms a common expression meaning:

• gets lost
• goes missing
• disappears

So you should understand bliver væk as a unit here, not word-by-word in a completely literal way.

Because nøgle is a common-gender noun in Danish:

• en nøgle = a key

For en-words, you use:

• min = my
• din = your
• sin/hans/hendes = his/her/one’s, depending on context

For et-words, you would use mit.

So:

• en nøgle → min nøgle
• not mit nøgle

Because min nøgle already makes the noun definite in meaning: my key.

In Danish, you normally do not use the suffixed definite ending when there is a possessive like min, din, hans, etc.

So:

• en nøgle = a key
• nøglen = the key
• min nøgle = my key

Not:

• min nøglen

This is because Danish is a verb-second (V2) language in main clauses.

When the hvis-clause comes first, it takes the first position in the sentence. Then the finite verb in the main clause must come next.

So the pattern is:

• Hvis ... , bliver jeg nervøs

Literally in terms of word order:

1. Hvis min nøgle bliver væk = fronted conditional clause
2. bliver = finite verb
3. jeg = subject
4. nervøs = complement

If the main clause stood alone, you would say:

• Jeg bliver nervøs.

But after an initial clause like Hvis min nøgle bliver væk, inversion happens:

• Hvis min nøgle bliver væk, bliver jeg nervøs.

Because a hvis-clause is a subordinate clause, and Danish subordinate clauses normally keep the subject before the finite verb:

• min nøgle = subject
• bliver = finite verb
• væk = complement/adverbial

So:

• Hvis min nøgle bliver væk

This is normal subordinate-clause word order.

A useful thing to remember is:

• in the subordinate clause, the subject typically comes before the verb
• in the main clause, Danish follows the V2 rule

Yes. Danish often uses the present tense for future meaning when the context makes it clear.

So Hvis min nøgle bliver væk, bliver jeg nervøs is present tense in form, but it naturally means something like:

• If my key gets lost, I get nervous
• If my key goes missing, I become nervous

English does something similar in many if-clauses:

• If it rains tomorrow, I’ll stay home.

You do not normally use a special future form after if in either language here.

The comma separates the conditional hvis-clause from the main clause:

• Hvis min nøgle bliver væk, = subordinate clause
• bliver jeg nervøs. = main clause

This comma is standard in written Danish.

Sometimes, yes, depending on context.

Nervøs often means:

• nervous
• anxious
• tense

In this sentence, nervous is the most straightforward translation, but anxious could also fit if the situation is more emotionally serious.

Yes:

• Jeg bliver nervøs, hvis min nøgle bliver væk.

This means the same thing.

The difference is mostly one of focus and style:

• Hvis min nøgle bliver væk, bliver jeg nervøs.
  The condition comes first.
• Jeg bliver nervøs, hvis min nøgle bliver væk.
  The reaction comes first.

Both are natural Danish.