Legen undersøker leddet.

Because legen is the definite singular form, meaning the doctor.

The base form is en lege = a doctor.
For many common-gender nouns in Bokmål, the definite singular is made by adding -en:

• en lege → legen
• en bil → bilen

So:

• lege = doctor
• legen = the doctor

Because leddet is the definite singular form, meaning the joint.

The noun is et ledd = a joint.
This is a neuter noun, and neuter nouns often take -et in the definite singular:

• et ledd → leddet
• et hus → huset

So:

• ledd = joint
• leddet = the joint

You would say:

En lege undersøker et ledd.

That is the indefinite version of the sentence.

Compare:

• En lege undersøker et ledd = a doctor examines a joint
• Legen undersøker leddet = the doctor examines the joint

Undersøker is the present tense of å undersøke.

So:

• å undersøke = to examine
• undersøker = examines / is examining

Norwegian present tense is often made by adding -r to the infinitive stem:

• å snakke → snakker
• å jobbe → jobber
• å undersøke → undersøker

Yes. In many contexts, Norwegian present tense can correspond to both the English simple present and present progressive.

So Legen undersøker leddet can mean:

• The doctor examines the joint
• The doctor is examining the joint

The exact meaning depends on context.

No. In Norwegian, the present-tense verb form stays the same for all persons.

For example:

• Jeg undersøker = I examine / am examining
• Du undersøker = you examine / are examining
• Han undersøker = he examines / is examining
• Legen undersøker = the doctor examines / is examining

This is much simpler than English, because you do not need a special -s form for he/she/it.

The sentence has the normal subject–verb–object order:

• Legen = subject
• undersøker = verb
• leddet = object

So:

Legen undersøker leddet.
= The doctor examines the joint.

This is the most neutral word order in a main clause.

Yes. Norwegian main clauses follow the V2 rule, which means the finite verb normally comes in the second position.

For example:

• I dag undersøker legen leddet. = Today, the doctor is examining the joint.

Notice what happened:

• I dag is now first
• undersøker must stay second
• legen comes after the verb

That is a very important Norwegian pattern.

Because Norwegian usually puts definiteness on the noun itself instead of using a separate word like English the.

So instead of saying something like the doctor with a separate article, Norwegian often says:

• legen = the doctor
• leddet = the joint

In this sentence, both nouns are definite, so both get definite endings.

No. Ledd can have several meanings, depending on context. It can mean things like:

• a joint in the body
• a link in a chain
• a part or section of something

But in Legen undersøker leddet, the medical meaning is the natural one: joint.