Barnet må adlyde reglene.

Because barn is a neuter noun: et barn. In Norwegian, the definite article is usually added to the end of the noun instead of being a separate word.

So:

• et barn = indefinite singular
• barnet = definite singular

Here, -et is the definite ending for a neuter singular noun.

Regel is usually an en-word, and its common forms are:

• en regel
• regelen
• regler
• reglene

So reglene is the definite plural form. The ending -ne marks definite plural.

Because må is a modal verb. After modal verbs such as må, kan, skal, vil, and bør, Norwegian uses the infinitive without å.

So:

• må adlyde = correct
• må å adlyde = wrong

It is the infinitive. After må, Norwegian uses the bare infinitive.

Compare:

• å adlyde = infinitive
• adlyder = present tense

In this sentence, adlyde is not conjugated because må is the finite verb.

Here må expresses necessity or obligation. It is the present-tense modal form of å måtte.

So in this kind of sentence, må means something like must or has to.

The sentence follows the normal Norwegian main-clause pattern:

subject + finite verb + infinitive + object

So here:

• Barnet = subject
• må = finite verb
• adlyde = infinitive
• reglene = object

If you move another element to the front, the finite verb still stays in second position:

• I dag må barnet adlyde reglene.

That second-position rule is very important in Norwegian.

In Norwegian, the is often built into the noun itself.

So instead of using a separate word, Norwegian often uses endings:

• barnet
• reglene

That is one of the biggest differences from English.

However, if there is an adjective, Norwegian often uses double definiteness:

• det lydige barnet
• de strenge reglene

Yes. Barn can be both singular indefinite and plural indefinite, depending on context.

But barnet is clearly singular definite.

Common forms are:

• et barn = singular indefinite
• barnet = singular definite
• barn = plural indefinite
• barna = plural definite

So the ending -et tells you this one is singular definite.

Yes, it is correct and understandable. But in everyday speech, many speakers would also say følge reglene.

There is a slight difference in feel:

• adlyde = obey, often a bit stricter or more about authority
• følge = follow, often more natural with rules in everyday situations

So adlyde reglene is fine, but følge reglene may sound more ordinary in some contexts.

You normally put ikke after the finite verb:

• Barnet må ikke adlyde reglene.

A very important warning: this means must not obey the rules.

It does not mean does not have to obey the rules. For that idea, Norwegian usually says:

• Barnet trenger ikke adlyde reglene.

This is a very common learner mistake.

A simple approximation is:

• Barnet ≈ BAR-ne
• må ≈ moh with a long vowel
• adlyde ≈ ahd-LY-deh
• reglene ≈ REG-le-ne

One useful detail: the written t in barnet is often not pronounced in standard speech.