Hon trodde först att däcket var helt tomt, men det var bara en liten punktering.

Däcket is the definite singular form of ett däck (a tire / the tire).

• ett däck = a tire
• däcket = the tire

In this sentence, the speaker is talking about a specific tire, so Swedish uses the definite form däcket.

Trodde is the past tense of tro, which means to think / to believe in the sense of having an opinion or assumption.

So:

• hon trodde att ... = she thought that ... / she believed that ...

A learner may wonder about tänkte, since English often uses thought for both ideas. But in Swedish:

• tro = to believe, to suppose
• tänka = to think, to reflect, to plan

Here the meaning is that she assumed the tire was empty/flat, so trodde is the natural choice.

Först here means at first / initially.

In a normal Swedish main clause, the finite verb usually comes in second position. That is why you get:

• Hon (subject)
• trodde (finite verb)
• först (adverb)

So Hon trodde först att ... is the standard word order.

English learners sometimes expect something like Hon först trodde ..., but that would not be the normal order in Swedish.

Att means that and introduces a subordinate clause.

So:

• Hon trodde att däcket var helt tomt
  = She thought that the tire was completely empty/flat

In spoken Swedish, att can sometimes be omitted, especially in casual speech, but keeping it is very common and perfectly natural.

Var is the past tense of är (is/was).

The sentence is reporting what she thought at that time, so Swedish uses past tense:

• Hon trodde ... att däcket var ...

This matches English well: She thought that the tire was ...

Using är would usually sound wrong here, because it would not fit the past-time perspective of trodde.

This is adjective agreement.

Däck is a neuter noun:

• ett däck

When an adjective describes a neuter singular noun, it often takes -t:

• tom → tomt

So:

• däcket var tomt = the tire was empty/flat

The word helt means completely.

So:

• helt tomt = completely empty

English would usually say completely flat for a tire, but Swedish often uses tomt in this kind of context.

That is a vocabulary difference between the languages.

In English, a tire is usually described as flat.
In Swedish, it is common to describe it as tomt (empty) because the idea is that the air is gone.

So while the literal meaning is empty, the natural meaning in context is something like:

• flat
• out of air

This is a good example of how direct word-for-word translation can sound odd even when the Swedish is completely normal.

Punktering means a puncture, and by extension it often refers to a flat tire caused by a puncture.

So in this sentence:

• en liten punktering = a small puncture

The idea is that the problem was not that the tire was totally empty for some major reason; it was just a minor puncture.

Because punktering is a common gender noun, not a neuter noun.

• en punktering
• punkteringen

Adjectives agree with that gender:

• en liten punktering

Compare:

• ett litet däck = a small tire
• en liten punktering = a small puncture

Also note that liten is irregular:

• en liten
• ett litet
• små in plural

Here det works naturally as it and refers to the situation/problem.

So:

• men det var bara en liten punktering = but it was only a small puncture

Swedish often uses det this way when identifying what the problem turned out to be.

You could expand the idea in English as:

• but it turned out to be only a small puncture

So det is not really replacing däcket in a strict mechanical way; it refers more generally to what the issue was.

Bara means only / just.

It makes the problem sound smaller or less serious:

• det var en liten punktering = it was a small puncture
• det var bara en liten punktering = it was only/just a small puncture

So bara softens the statement and shows relief: the problem was not as bad as she first thought.

In a sentence like this, yes: men simply means but.

It connects two contrasting ideas:

• first she believed the tire was completely empty
• but in reality it was only a small puncture

So men marks that contrast very clearly.

Yes, especially in spoken or informal Swedish, you may hear:

• Hon trodde först däcket var helt tomt ...

But with att:

• Hon trodde först att däcket var helt tomt ...

This is clearer and more standard, especially for learners. If you are writing carefully, keeping att is a good choice.

Yes. The first clause follows normal Swedish V2 word order, where the finite verb comes second:

• Hon = first element
• trodde = second element

Then the subordinate clause after att has its own structure:

• att däcket var helt tomt

The second main clause also follows normal word order:

• men det var bara en liten punktering

So this sentence is a useful example of:

• a main clause
• a subordinate clause introduced by att
• another main clause joined by men