Hvis reservedækket også er fladt, må vi ringe til mekanikeren.

Because Danish, like German, very often forms nouns as compound words.

• reserve = spare / reserve
• dæk = tire
• reservedæk = spare tire

In the sentence, it appears as reservedækket, which is the definite form: the spare tire.

So:

• et reservedæk = a spare tire
• reservedækket = the spare tire

Danish usually writes these compounds as one word, not as two separate words.

Danish often makes nouns definite by adding the definite article as an ending.

For neuter nouns like et dæk, the definite singular is usually -et:

• et dæk = a tire
• dækket = the tire

The same happens with the compound noun:

• et reservedæk = a spare tire
• reservedækket = the spare tire

So the ending -et works like English the.

Because the adjective has to agree with the noun.

Dæk is a neuter noun because it takes et:

• et dæk

When an adjective is used with a neuter singular noun after er, it often takes -t:

• Dækket er fladt = The tire is flat

Compare:

• en bil er flad — not natural semantically, but grammatically it shows common gender
• et dæk er fladt

So fladt is used because reservedækket comes from et reservedæk, a neuter noun.

Here må means something like must, have to, or will have to.

So:

• må vi ringe = we must / have to call

This is one of the common meanings of må in Danish. Depending on context, må can also mean may / be allowed to, but in this sentence it clearly expresses necessity.

So the idea is:

• If the spare tire is also flat, we’ll have to call the mechanic.

This is because of normal Danish word order.

The sentence begins with a subordinate clause:

• Hvis reservedækket også er fladt = If the spare tire is also flat

After that clause comes the main clause. In Danish main clauses, the finite verb normally comes in second position. Since the if-clause takes the first position, the verb of the main clause comes next:

• Hvis ..., må vi ...

So:

• Hvis reservedækket også er fladt, må vi ringe til mekanikeren.

If the main clause stood alone, it would be:

• Vi må ringe til mekanikeren.

But after the fronted hvis-clause, Danish switches to:

• ..., må vi ...

This is a very important Danish word-order pattern.

Også means also / too.

In this sentence:

• Hvis reservedækket også er fladt = If the spare tire is also flat

Here også modifies er fladt, meaning that the spare tire is flat as well—probably in addition to another tire already being flat.

Its position is natural in Danish and helps show that being flat is the extra thing being added.

A learner may also notice that Danish adverbs often appear in positions that do not match English exactly, so it is best to learn common patterns from full sentences.

Because the verb ringe takes til when you call someone.

• at ringe til nogen = to call someone

So:

• Vi ringer til mekanikeren = We call the mechanic

This is different from English, where call someone does not need a preposition.

You should learn it as a fixed pattern:

• ringe til en ven
• ringe til politiet
• ringe til mekanikeren

Mekanikeren means the mechanic.

In context, this often suggests a specific mechanic the speakers have in mind, or simply the mechanic as the natural person to call in that situation.

Compare:

• en mekaniker = a mechanic
• mekanikeren = the mechanic

Danish often uses the definite form where English also uses the.

Because hvis is the normal word for if in conditional sentences.

This sentence is a condition:

• If the spare tire is also flat, ...

So Danish uses:

• hvis

A useful basic distinction is:

• hvis = if, in conditions
• om = whether

So here hvis is the correct choice because the sentence is not about whether something is true, but about a condition and its result.

The sentence has two parts:

1. Hvis reservedækket også er fladt

   • subordinate clause
   • gives the condition
   • If the spare tire is also flat

2. må vi ringe til mekanikeren

   • main clause
   • gives the result
   • we’ll have to call the mechanic

So the pattern is:

• Hvis + condition, main clause

This is a very common Danish sentence pattern for expressing conditions.