Hun finder målebåndet igen, fordi vi vil være sikre på, at den her reol passer.

Because målebåndet is the definite form: the measuring tape.

In Danish, singular nouns usually make the definite form by adding an ending:

• et målebånd = a measuring tape
• målebåndet = the measuring tape

So Hun finder målebåndet igen means she is finding a specific measuring tape, not just any measuring tape.

Yes. Målebånd is a compound noun:

• måle = measure
• bånd = tape/band

Danish makes compounds as one word, and the definite ending goes on the whole compound, which means it appears on the last part:

• et målebånd
• målebåndet

That is very normal in Danish. Compare the same pattern in many other compounds.

That is the normal placement here.

In a simple main clause, Danish often has:

• subject + verb + object + adverb

So:

• Hun
  • finder
    • målebåndet
      • igen

This gives She finds the measuring tape again.

If you move igen, the sentence may sound odd or the emphasis may change. For a learner, verb + object + igen is a very good default pattern to recognize here.

Those commas mark the start of subordinate clauses:

• fordi vi vil være sikre på ...
• at den her reol passer

In Danish, this is correct if the writer is using start comma. Modern Danish allows two comma systems, so you may also see the same sentence written without those starting commas.

So these commas are not strange; they reflect one accepted Danish punctuation style.

Yes, subordinate clauses in Danish follow subordinate-clause word order, but in this sentence the difference is not very visible.

You have:

• fordi vi vil være sikre på
• at den her reol passer

These look similar to main-clause order because there is no word like ikke in them.

The difference becomes clearer with negation:

• fordi vi ikke vil være sikre ... would be wrong
• correct: fordi vi ikke vil ...
• correct: at den her reol ikke passer

So after fordi and at, words like ikke normally come before the finite verb.

Because vil is a modal verb, and modal verbs are followed by the infinitive.

So:

• vi vil være sikre = we want to be sure / we will be sure depending on context

Not:

• vi vil er sikre ❌

Here vil means something like want to rather than a simple future marker.

Because the subject is vi = we, which is plural.

When sikker is used as a predicate adjective, it agrees in number:

• jeg er sikker = I am sure
• hun er sikker = she is sure
• vi er sikre = we are sure

So sikre is the correct plural form here.

Because at være sikker på is the normal Danish expression for to be sure of/about or to be sure that.

You can have:

• være sikker på noget = be sure about something
• være sikker på, at ... = be sure that ...

So in this sentence:

• vi vil være sikre på, at den her reol passer

The på belongs naturally with sikker in this construction.

Here at means that and introduces the content clause:

• vi vil være sikre på, at den her reol passer
• we want to be sure that this bookcase fits

In English, that is sometimes omitted. In Danish, at is usually kept in this kind of sentence, especially after a phrase like sikker på.

Because den her reol means this bookcase / this shelf unit, and that construction uses:

• den/det + her + indefinite noun

So:

• den her reol = this bookcase
• not den her reolen ❌

The noun stays indefinite because the definiteness is already expressed by den her.

By contrast:

• reolen = the bookcase

So den her reol and reolen are different structures.

Because reol is a common-gender noun in Danish, and common-gender nouns take den.

• en reol
• den her reol

If the noun were a neuter noun, you would use det:

• et bord
• det her bord

So the choice between den and det depends on the noun’s grammatical gender.

Yes. Denne reol is also correct and means the same basic thing: this bookcase.

But there is a difference in style:

• den her reol = very common in everyday speech
• denne reol = a bit more formal or written

A learner will hear den her very often in spoken Danish.

Here passer means fits.

The verb at passe has several meanings in Danish, for example:

• fit
• suit
• match
• take care of

In this sentence, it means that the bookcase is the right size or dimensions for the place in question.

So:

• at den her reol passer = that this bookcase fits

It is not related to English pass in the usual sense of go by or succeed in a test.