Hvis torden og lyn kommer tættere på, går vi ind og lukker døren til altanen.

Because Danish main clauses follow the V2 rule: the finite verb normally comes in the second position.

Here, the whole Hvis torden og lyn kommer tættere på clause comes first. That means the next thing must be the finite verb of the main clause, so you get:

Hvis ..., går vi ind ...

If the sentence started directly with the main clause, it would be:

Vi går ind og lukker døren til altanen.

So the word order changes because the if-clause is placed first.

Danish often uses the present tense for future meaning when the context already makes it clear.

A condition like hvis naturally points to something that may happen later, so present tense is completely normal:

• Hvis det regner, bliver vi hjemme.
• Hvis torden og lyn kommer tættere på, går vi ind ...

English often does something similar: If it rains, we stay home or we’ll stay home.

You can use forms with vil in some cases, but simple present is usually the most natural neutral choice here.

Hvis means if and is used for a condition: maybe it happens, maybe it does not.

Når means when and usually suggests that the speaker sees it as something expected or certain to happen at some point.

So:

• Hvis torden og lyn kommer tættere på = if thunder and lightning come closer
• Når torden og lyn kommer tættere på = when thunder and lightning come closer

In this sentence, hvis fits because it is a conditional situation.

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

The expression komme tættere på is a very common Danish way to say come/get closer. The på is part of the construction.

Literally, it is something like come closer to. The reference point is often left unstated because it is obvious from context: closer to us, to here, to the building, and so on.

So kommer tættere på works as a natural unit.

Because ind shows movement inward, while inde describes location.

Compare:

• Vi går ind. = we go inside
• Vi er inde. = we are inside

So in this sentence, går vi ind is correct because the people are moving from outside to inside.

Because torden and lyn are being used in a general, non-specific way.

Danish often uses bare nouns like this for weather and natural phenomena:

• regn
• sne
• tåge
• torden og lyn

The sentence is not talking about one specific thunderclap or one specific flash of lightning. It means thunder and lightning as a general weather event, so no article is needed.

Because the first part is a subordinate clause introduced by hvis, and Danish normally places a comma between a subordinate clause and the main clause.

So the structure is:

• subordinate clause: Hvis torden og lyn kommer tættere på
• main clause: går vi ind og lukker døren til altanen

That is why the comma appears there.

Both are possible.

• døren til altanen = the door to the balcony
• altandøren = the balcony door

The version in the sentence is a little more descriptive and transparent for a learner, while altandøren is a standard Danish compound noun.

So this is not about right vs. wrong. It is mainly a choice of expression.

Because it is definite: it means the balcony, not a balcony.

In Danish, definiteness is often shown by adding an ending to the noun:

• en altan = a balcony
• altanen = the balcony

Here, the balcony is understood to be a specific known one, probably the balcony belonging to the home or building the speakers are in.

Because the same subject, vi, applies to both verbs:

• går
• lukker

Danish, like English, usually does not repeat the subject when two verbs are joined by og and clearly have the same subject:

går vi ind og lukker døren ...

This is the same idea as English we go inside and close the door.

You could repeat vi in some contexts for emphasis or clarity, but it is not needed here.

No, Danish verbs do not change for person and number the way English verbs sometimes do.

For example, in the present tense:

• jeg går
• du går
• han går
• vi går
• de går

The verb form stays the same.

So in this sentence, there is no special plural form because the subject is torden og lyn, and there is no special we form in går vi or lukker. Danish verbs are much simpler in this way than English verbs.