Læknirinn greinir vandamálið.

Those endings are the suffixed definite article in Icelandic. Instead of using a separate word like English the, Icelandic often attaches it to the noun.

• læknir = doctor

• læknirinn = the doctor

• vandamál = problem
• vandamálið = the problem

The form of the article changes depending on the noun’s gender, number, and case.

Because læknir means a doctor / doctor, while læknirinn means the doctor.

So:

• Læknir greinir vandamál. = A doctor diagnoses a problem or Doctor diagnoses problem in a very bare grammatical sense
• Læknirinn greinir vandamálið. = The doctor diagnoses the problem

The sentence you have uses the definite forms for both nouns.

Greinir is the present tense, 3rd person singular form of the verb greina.

So it matches a subject like:

• hann = he
• hún = she
• það = it
• or a singular noun like læknirinn

In this sentence:

• læknirinn = singular subject
• therefore the verb is greinir = diagnoses / identifies / analyzes

The dictionary form is greina.

That is the infinitive, equivalent to English to diagnose, to identify, to analyze, or to distinguish, depending on context.

A learner often sees:

• að greina = to diagnose / to identify / to analyze

Then in the sentence it becomes:

• greinir = diagnoses / identifies / analyzes

No. Greina has a fairly broad range of meanings, and the exact translation depends on context.

Common meanings include:

• diagnose
• identify
• analyze
• distinguish

With læknirinn as the subject, the medical meaning diagnoses is the most natural one.

So in this sentence, greinir is best understood as diagnoses.

Because it is the thing being acted on.

In Læknirinn greinir vandamálið:

• læknirinn = the one doing the action → the subject
• greinir = the action → the verb
• vandamálið = the thing being diagnosed → the object

So the structure is:

• The doctor
  • diagnoses
    • the problem

That is the same basic pattern as English subject + verb + object.

It is functioning as the direct object, so it is in the accusative case.

A useful thing to know is that for many neuter nouns, the nominative and accusative forms are often the same in the singular. So even though vandamálið is accusative here, it looks the same as the nominative definite form.

That can be confusing at first, but it is very normal in Icelandic.

Yes, it matters a lot in Icelandic.

• læknir is masculine
• vandamál is neuter

Gender affects things like:

• the form of the definite article
• adjective endings
• pronouns
• sometimes declension patterns

That is why the definite forms are different:

• læknirinn = masculine definite singular
• vandamálið = neuter definite singular

Yes. Læknirinn greinir vandamálið uses the normal, straightforward order:

• Subject – Verb – Object

So:

• Læknirinn = subject
• greinir = verb
• vandamálið = object

This is a very common neutral word order in Icelandic, especially in simple main clauses.

However, Icelandic can vary word order more than English because case endings help show grammatical roles.

A rough pronunciation guide for an English speaker is:

• Læknirinn ≈ LIGHK-ni-rin
• greinir ≈ GRAY-nir
• vandamálið ≈ VAHN-da-mau-lith

A few important notes:

• æ sounds roughly like eye
• ei sounds roughly like ay
• ð is like the th in this
• Icelandic stress usually falls on the first syllable

So the sentence stress pattern is roughly:

LÆKnirinn GREInir VANdamálið

That is just how the language normally expresses definiteness. Icelandic usually uses a suffixed article, attached to the noun itself.

So instead of:

• English: the doctor
• Icelandic: læknirinn

And instead of:

• English: the problem
• Icelandic: vandamálið

This is one of the first big structural differences English speakers notice when learning Icelandic.

Yes, depending on context, that can be a reasonable translation.

Because greina is broader than just medical diagnose, the sentence could also be understood as:

• The doctor identifies the problem
• The doctor analyzes the problem

But if the intended meaning is medical, The doctor diagnoses the problem is the most natural English translation.

You would usually remove the definite article from læknirinn:

• Læknir greinir vandamálið.

That gives:

• læknir = a doctor / doctor
• vandamálið = the problem

So the meaning becomes:

• A doctor diagnoses the problem

If you also wanted a problem, you would remove the article from vandamálið too:

• Læknir greinir vandamál.

That would mean:

• A doctor diagnoses a problem or Doctor diagnoses problem in a very stripped-down grammatical sense

In actual usage, context helps make the intended meaning clear.