Hun går ind, når det begynder at regne.

Går is the present tense of gå.

• gå = to go / to walk
• går = goes / is going / walks

A useful thing to remember: Danish verbs do not change for person the way English verbs do. So you get:

• jeg går
• du går
• hun går
• vi går

It is always går in the present tense.

Because ind adds the idea of movement inward / inside.

• Hun går = she walks / she goes
• Hun går ind = she goes in / she goes inside

So ind is important here. Without it, the sentence would lose the idea that she moves indoors.

In Danish, words like ind, ud, op, and ned often act as particles that combine with a verb.

So gå ind works a bit like an English phrasal verb:

• gå ind = go in
• gå ud = go out
• stå op = get up / stand up

They are normally written as separate words in sentences like this.

Når means when in the sense of whenever or at the time that.

In this sentence, it suggests a general or repeated situation:

• every time it starts raining, she goes inside

That is why når is natural here.

These are easy to mix up.

• når = when, often for repeated situations or future time
• hvis = if
• da = when, usually about a specific event in the past

So:

• Hun går ind, når det begynder at regne. = she goes inside when it starts raining
• Hun går ind, hvis det begynder at regne. = she goes inside if it starts raining
• Hun gik ind, da det begyndte at regne. = she went inside when it started raining

This det is a dummy subject, just like English it in weather expressions.

English says:

• it is raining
• it is snowing
• it starts to rain

Danish does the same:

• det regner
• det sner
• det begynder at regne

So det does not refer to a real thing. It is just the grammatical subject needed for the sentence.

Because begynde usually takes at + infinitive.

So:

• begynder at regne = begins to rain / starts to rain

This is very similar to English to in starts to rain.

Other examples:

• begynder at spise = begins to eat
• begynder at arbejde = begins to work

Because after at, Danish normally uses the infinitive form.

• regne = infinitive
• regner = present tense

So:

• det begynder at regne = correct
• det begynder at regner = incorrect

Compare:

• Det regner. = It is raining.
• Det begynder at regne. = It starts to rain.

It can often be understood as a general/habitual statement, and that is probably the most natural reading here:

• whenever it starts raining, she goes inside

But Danish present tense can also be used for the future, especially with time words and clauses like når.

So depending on context, it could mean either:

• a general habit, or
• what she will do next time it rains

Yes. You can also say:

Når det begynder at regne, går hun ind.

That means the same thing.

But notice the word order in the main clause after the opening når-clause:

• Når det begynder at regne, går hun ind.

Not:

• Når det begynder at regne, hun går ind.

This happens because Danish main clauses normally follow the verb-second pattern.

Because når det begynder at regne is a subordinate clause.

In Danish, many writers use a comma before subordinate clauses:

• Hun går ind, når det begynder at regne.

But Danish also allows a system without this start comma, so you may also see:

• Hun går ind når det begynder at regne.

Both can be correct, depending on the comma style being used.

Yes, it is a very natural and common way to say it.

• Det begynder at regne = It starts to rain

You will hear and see this a lot in everyday Danish. It is the standard, idiomatic expression for this idea.