Testamentet gjelder dødsboet.

Because both nouns are singular definite neuter nouns.

In Norwegian, the definite article is usually attached to the end of the noun:

• et testament = a will

• testamentet = the will

• et dødsbo = an estate of a deceased person
• dødsboet = the estate

So -et here works like English the, but as a suffix instead of a separate word.

The dictionary form is å gjelde.

Here, gjelder is the present tense:

• å gjelde = to concern / to apply to / to be valid for
• gjelder = concerns / applies to / is valid for

So this sentence is in the present tense, which is very common in Norwegian for general statements and legal facts.

Because Norwegian usually puts the definite article onto the noun itself.

So instead of saying something like:

• det testament
• det dødsbo

Norwegian normally says:

• testamentet
• dødsboet

That suffix already contains the idea of the.

Yes. Dødsboet comes from the compound noun dødsbo.

It is built like this:

• død = death
• linking -s-
• bo = estate / property holding / estate of a person

So:

• dødsbo = estate left by someone who has died

Then the definite ending is added:

• dødsboet = the estate

A useful thing to remember about Norwegian compounds is that the last part is the main noun. Here, the main noun is bo, so the whole compound has the same gender as bo, which is neuter.

Because this is normal subject–verb–object word order in a main clause.

The sentence is:

• Testamentet = subject
• gjelder = verb
• dødsboet = object

So the pattern is:

• Subject + Verb + Object

That is the most basic word order in Norwegian main clauses when nothing else has been moved to the front.

In this sentence, gjelde is used without a preposition and takes a direct object:

• Testamentet gjelder dødsboet.

That is natural when gjelde means something like concern or relate to.

Norwegian also has examples with gjelde for, especially when the meaning is more like be valid for:

• Reglene gjelder for alle. = The rules apply to everyone.

So both patterns exist, but they are not always interchangeable. In your sentence, the direct-object pattern is the natural one.

In Norwegian, testament is the normal legal word for a will.

So unlike in English, where testament may sound old-fashioned, biblical, or formal outside legal contexts, Norwegian testament is the standard legal term.

That makes this sentence sound natural in a legal or administrative context.

You could, but it would not mean quite the same thing.

• gjelder = concerns, applies to, relates to in a legal or formal sense
• handler om = is about, deals with, has as its topic

So:

• Testamentet gjelder dødsboet sounds legal and precise.
• Testamentet handler om dødsboet sounds more like the will is about the estate, which is possible but less exact in this kind of context.

For legal language, gjelder is the better choice.

Yes. Both nouns are definite:

• testamentet = the specific will
• dødsboet = the specific estate

So the sentence is not talking about wills and estates in general. It refers to a particular will and a particular estate.

If you wanted a more general statement, you would use indefinite forms:

• Et testament gjelder et dødsbo.

That would sound more general, though the original definite sentence is much more natural in a real legal context.

A rough guide is:

• gjelder ≈ YEL-der

A few helpful points:

• The gj at the beginning is pronounced like a y sound for many learners.
• The first syllable is stressed: GJEL-der
• The e in the first syllable is short.

You do not need a perfect accent to be understood, but pronouncing the start more like y than a hard English g will help a lot.