Læknirinn undirbýr aðgerðina.

The dictionary forms are:

• læknir = doctor
• undirbúa = to prepare
• aðgerð = operation, procedure, surgery

In the sentence, these words are inflected:

• læknirinn = læknir
  • the
• undirbýr = a present-tense form of undirbúa
• aðgerðina = aðgerð
  • the, in the object form

Because Icelandic usually adds the definite article directly to the end of the noun instead of using a separate word like English the.

So:

• læknir = doctor
• læknirinn = the doctor

Here, -inn is the definite ending for this form of a masculine noun.
Also, læknirinn is the subject of the sentence, so it is in the nominative singular.

Because aðgerðina is the direct object of the verb undirbýr.

The verb undirbúa takes an object, and that object is normally in the accusative case. So the noun aðgerð has to appear in its accusative singular definite form:

• aðgerð = operation / procedure
• aðgerðin = the operation (nominative singular)
• aðgerðina = the operation (accusative singular)

So in this sentence:

• læknirinn is the subject
• aðgerðina is the object

That is why the two nouns have different endings even though both mean the ...

Undirbýr is the 3rd person singular present tense of undirbúa.

So it means:

• he/she prepares
• is preparing

Because the subject is læknirinn = the doctor (singular), the verb is also singular.

A useful mini-paradigm is:

• ég undirbý = I prepare / am preparing
• þú undirbýrð = you prepare / are preparing
• hann/hún undirbýr = he/she prepares / is preparing

It can mean either, depending on context.

Icelandic present tense often covers both:

• The doctor prepares the operation
• The doctor is preparing the operation

English usually forces you to choose between simple present and present continuous, but Icelandic often does not. Context tells you which reading is intended.

The sentence has the basic Icelandic word order:

• Subject + Verb + Object

So:

• Læknirinn = subject
• undirbýr = verb
• aðgerðina = object

This is a very natural and neutral order.

However, Icelandic also has verb-second word order in main clauses. That means if you move something else to the front, the finite verb usually still stays in second position. For example:

• Núna undirbýr læknirinn aðgerðina.
  = Now the doctor is preparing the operation.

The nouns have different grammatical genders:

• læknir is masculine
• aðgerð is feminine

Gender matters in Icelandic because it affects:

• the form of the definite article
• adjective endings
• some pronouns and other agreement patterns

That is why the definite forms are different:

• læknirinn = masculine
• aðgerðina = feminine accusative

So the endings are not random; they reflect the noun’s gender and case.

A few letters may stand out to an English speaker:

• æ in læknirinn sounds roughly like the vowel in eye
• ð in aðgerðina is like the th in this
• ý in undirbýr sounds like a long ee, similar to see

Very rough pronunciation help:

• læknirinn ≈ LYE-knih-rin
• undirbýr ≈ UN-dir-beer
• aðgerðina ≈ ATH-gerth-i-na

These are only approximations, but they can help you get started.

Very often, yes. Icelandic commonly uses a suffixed definite article, meaning the is attached to the noun:

• læknir = doctor

• læknirinn = the doctor

• aðgerð = operation
• aðgerðin / aðgerðina = the operation, depending on case

There is also a separate definite word, hinn, but that is used in more specific situations, often with adjectives or in formal/literary style. For everyday sentences like this one, the attached article is the normal pattern.

You can tell from a combination of word order and case endings.

• læknirinn is in the nominative, which is the usual case for the subject
• aðgerðina is in the accusative, which is the usual case for a direct object

So even if you only looked at the forms, you could identify:

• the doctor = the one doing the action
• the operation = the thing being prepared

That is one of the advantages of Icelandic case marking: the endings give grammatical information that English usually shows mostly through word order.