Han skjuler stoltheten sin.

Because Norwegian usually makes the noun definite when the possessive comes after it.

• stolthet = pride
• stoltheten = the pride

So stoltheten sin is the normal Norwegian way to say his own pride.
This is one of those places where Norwegian structure does not match English word-for-word.

Because sin is a reflexive possessive. That means it refers back to the subject of the sentence.

In Han skjuler stoltheten sin, the subject is Han, so sin means his own.

Compare:

• Han skjuler stoltheten sin = He hides his own pride.
• Han skjuler stoltheten hans = He hides another man's pride.

So this sentence uses sin because the pride belongs to the same person as han.

Norwegian allows two main patterns:

• stoltheten sin
• sin stolthet

Both can mean his own pride.

The version with the possessive after the noun is very common and very natural in everyday Norwegian. When you use that pattern, the noun normally becomes definite:

• stoltheten sin

If the possessive comes before the noun, the noun is usually indefinite:

• sin stolthet

Yes. That is also correct.

Both of these are grammatical:

• Han skjuler stoltheten sin
• Han skjuler sin stolthet

The first version often sounds more neutral and everyday.
The second can sound a bit more formal, literary, or slightly emphatic.

So as a learner, it is useful to recognize both patterns.

Because the possessive has to match the noun it belongs to.

stolthet is a singular common-gender noun, so the correct form is sin.

The forms are:

• sin = singular common gender
• sitt = singular neuter
• sine = plural

Examples:

• boka si = his/her own book
• huset sitt = his/her own house
• vennene sine = his/her own friends

So here, stoltheten sin is correct because stolthet is singular common gender.

skjuler is the present tense of å skjule.

• å skjule = to hide
• skjuler = hide / hides

Norwegian verbs do not change according to person the way English verbs do. So you get:

• jeg skjuler
• du skjuler
• han skjuler

The verb form stays the same.

It follows the normal Norwegian main-clause pattern:

• Han = subject
• skjuler = verb
• stoltheten sin = object

So it is basically subject + verb + object.

Norwegian also has the V2 rule, which means the finite verb usually stays in the second position in main clauses. For example:

• I dag skjuler han stoltheten sin.

Here I dag comes first, but skjuler still stays in the second position.

Two things often surprise English speakers:

• skj in skjuler is usually pronounced like a single sh sound.
• th in stolthet is not the English th sound. It is pronounced like a normal t.

So:

• skjuler starts with something like sh-
• stolthet has a regular t sound in the middle

That means you should not pronounce skj as separate s-k-y sounds, and you should not pronounce th like in think or this.