Spænd dit bælte, før du går ud af huset.

Because spænd is the imperative form of at spænde. It is used for commands:

• Spænd dit bælte! = Fasten your belt!

In Danish, commands usually use the verb by itself, without a subject like du.

By contrast:

• du spænder = you fasten / you are fastening

So spænd is not a statement; it is an instruction.

Here, spænde means something like fasten, buckle, or tighten.

With bælte, it usually means securing the belt properly. The best English translation depends on context:

• fasten your belt
• buckle your belt
• tighten your belt

So the Danish verb is a little broader than any one single English verb.

Because bælte is a neuter noun in Danish:

• et bælte = a belt

For your in the singular, Danish changes the form to match the noun:

• din for common-gender nouns
• dit for neuter nouns
• dine for plural nouns

Examples:

• din bil = your car
• dit bælte = your belt
• dine sko = your shoes

So dit bælte is correct because bælte is an et-word.

Because Danish normally does not use an article together with a possessive in this kind of phrase.

So you say:

• dit bælte = your belt

not:

• det dit bælte

This is similar to English, where you say your belt, not the your belt.

Før means before and introduces the clause:

• før du går ud af huset = before you go out of the house

Yes, inden can often be used in similar situations:

• Inden du går ud af huset

That would also be natural Danish. In this sentence, før is a very normal and straightforward choice.

It breaks down like this:

• går = go / walk
• ud = out
• af = of / out of
• huset = the house

Together, gå ud af means go out of.

So:

• du går ud af huset = you go out of the house

This kind of combination is very common in Danish. The small words like ud, ind, op, ned, af often work together to show direction.

Because huset is the definite form:

• hus = house
• huset = the house

In this sentence, a specific house is meant — the one the person is in or the one already understood from context.

So:

• ud af huset = out of the house

If you said ud af et hus, that would mean out of a house, which is more indefinite.

Because før introduces a subordinate clause, and in this kind of clause Danish normally has:

• subject + verb

So:

• før du går = correct

not:

• før går du

Går du is the kind of order you often get in a question or in a main clause with inversion. That is not what is happening here.

Because før du går ud af huset is a subordinate clause, and Danish often puts a comma before such clauses.

So this is normal:

• Spænd dit bælte, før du går ud af huset.

You may also see:

• Spænd dit bælte før du går ud af huset.

That is also accepted in modern Danish, depending on which comma system the writer is using. So the comma here is normal, but you may encounter both versions.

A rough guide:

• spænd: the æ is somewhat like the vowel in English cat, but not exactly
• bælte: same æ sound in the first syllable
• før: the ø sound does not exist in standard English; it is one of the vowels English speakers usually need to practice
• går: the å is an aw/o-type sound

For many English speakers, the vowels in æ, ø, and å are the hardest part of this sentence.