Den regel gælder også i skolen.

Because regel is a common-gender noun in Danish. Common-gender nouns take den in the singular, while neuter nouns take det.

• en regel = a rule
• den regel = that rule / the rule (depending on context)

So det regel would be incorrect.

Den regel and reglen are not exactly the same.

• reglen = the rule
• den regel = usually that rule

In Danish, using den + noun often points to a specific noun in a more demonstrative way, like English that. So Den regel gælder også i skolen most naturally feels like That rule also applies in school / at school.

If you said Reglen gælder også i skolen, that would more naturally mean The rule also applies in school.

Here gælder means applies or is valid.

The verb at gælde is often used for rules, laws, conditions, prices, agreements, and similar things.

Examples:

• Reglen gælder for alle. = The rule applies to everyone.
• Tilbuddet gælder kun i dag. = The offer is only valid today.
• Det gælder ikke længere. = That no longer applies.

So in this sentence, gælder is the natural verb for saying that a rule is in force in a certain context.

That is because Danish main clauses usually follow the verb-second pattern, often called V2. The finite verb typically comes early in the sentence, and adverbs like også often come after it.

So:

• Den regel = subject
• gælder = finite verb
• også = adverb
• i skolen = prepositional phrase

This word order is very normal in Danish:

• Han kommer også i morgen. = He is also coming tomorrow.
• Det gælder også her. = That also applies here.

English learners sometimes want to put also earlier, but Danish usually keeps også after the verb in a simple main clause like this.

Yes, but it changes the emphasis.

• Den regel gælder også i skolen = the most neutral version; the rule applies there too
• Også den regel gælder i skolen = that rule too applies in school

So the first sentence emphasizes also in school, while the second emphasizes that rule as well.

Danish word order is often used to shift emphasis like this.

Danish often uses the definite form in places where English uses no article.

So:

• i skolen literally looks like in the school
• but it can correspond to English in school or at school, depending on context

This is normal Danish usage. Another example:

• på hospitalet = at the hospital
• i kirken = in church / at church

So even though English may say in school without the, Danish often prefers i skolen.

Both can occur, but they are not always felt the same way.

• i skolen often focuses on being in the school setting / inside school / in school
• på skolen often focuses on being at the school as a place or institution

In many situations, the difference is small, and usage can vary by region or context. In this sentence, i skolen sounds very natural for the idea that the rule applies within the school context.

So:

• Den regel gælder også i skolen = That rule also applies in school / within the school setting
• Den regel gælder også på skolen = That rule also applies at the school

It is singular definite.

The noun is:

• en skole = a school
• skolen = the school

The ending -en is the definite singular ending for many common-gender nouns.

Plural forms are:

• skoler = schools
• skolerne = the schools

So i skolen is singular: in the school / at school.

A rough guide is GEL-der, but the Danish sounds are not exactly like English.

A few points:

• gæ- has a vowel somewhat like the e in get, but Danish pronunciation is more open and relaxed
• ld in Danish is often pronounced more softly than English learners expect
• the final -er is usually a weak, unstressed ending

So the word is not pronounced exactly as it looks to an English speaker. If you are learning pronunciation, it is worth listening to native audio for gælder, because Danish spelling and pronunciation often do not match in an obvious way.

Yes. In English, both in school and at school may work depending on the situation.

The Danish phrase i skolen can cover the general school context, so English may choose different prepositions depending on what sounds most natural:

• That rule also applies in school
• That rule also applies at school

If the meaning shown to the learner uses one of these, that does not necessarily mean the other is wrong. It is often just a matter of natural English phrasing.

Yes, very normal. It follows a common Danish structure:

subject + finite verb + adverb + prepositional phrase

Here that gives:

• Den regel = subject
• gælder = finite verb
• også = adverb
• i skolen = place/context phrase

This is a useful model for many other sentences:

• Den lov gælder også i Danmark. = That law also applies in Denmark.
• Det gælder også på arbejdet. = That also applies at work.
• Reglen gælder stadig i dag. = The rule still applies today.