Hvis torden og lyn kommer tættere på, går vi ind i huset.

Hvis means if and introduces a conditional clause.

So:

• Hvis torden og lyn kommer tættere på = If the thunder and lightning come closer

This first part sets the condition for what happens in the main clause.

This is because Danish follows the verb-second rule in main clauses.

When the sentence begins with something other than the subject — here, the whole Hvis... clause — the finite verb comes before the subject in the main clause:

• Hvis ..., går vi ind i huset.

The main clause is:

• går vi ind i huset

If you started with the subject instead, you would say:

• Vi går ind i huset, hvis torden og lyn kommer tættere på.

Both are correct; the word order changes because of what comes first.

Danish often uses the present tense to talk about the future when the context makes the future meaning clear.

So:

• Hvis torden og lyn kommer tættere på, går vi ind i huset.

means something like:

• If the thunder and lightning get closer, we’ll go into the house.

English often uses present tense in the if-clause too, but Danish commonly keeps the whole sentence in present tense where English might use will in the main clause.

Tættere is the comparative of tæt, so it means closer.

På belongs with the expression here, so komme tættere på means:

• to come closer
• to move nearer

So:

• kommer tættere på = comes closer / gets nearer

This is a very natural way to describe something approaching.

Because komme tættere på is a fixed combination in Danish.

You should think of it as a unit:

• komme = come
• tættere på = closer / nearer

The på is necessary here. Without it, the phrase would sound incomplete or different in meaning.

Because ind i expresses movement into something.

Compare:

• vi er i huset = we are in the house
• vi går ind i huset = we go into the house

So:

• i huset describes location
• ind i huset describes motion from outside to inside

That is why ind is important here.

Huset means the house.

In Danish, the definite article is often attached to the end of the noun:

• et hus = a house
• huset = the house

So the sentence refers to a specific house, probably one already understood from the situation.

No. In Danish, nouns like torden and lyn are often used without an article when speaking generally.

So:

• torden og lyn = thunder and lightning

This is similar to English, where you would also normally say thunder and lightning, not the thunder and the lightning, unless you mean something very specific.

Yes, in standard Danish writing you normally place a comma between the subordinate clause and the main clause:

• Hvis torden og lyn kommer tættere på, går vi ind i huset.

The comma marks the boundary between:

• the if-clause
• the main clause

This is standard and helpful for reading.

Sometimes yes, but tættere på is very natural here.

For example:

• kommer nærmere = comes nearer
• kommer tættere på = comes closer

Both can work, but tættere på strongly suggests approaching a specific place or person. In this sentence, it fits very well with the idea of a storm moving closer to where we are.