Aðgerðin hefst á morgun.

Because Icelandic usually puts the definite article on the end of the noun instead of using a separate word like the.

• aðgerð = operation / action / measure
• aðgerðin = the operation / the action / the measure

Here, -in is the definite ending for this form of the noun. So aðgerðin is simply the definite form of aðgerð.

It is nominative singular.

That is because aðgerðin is the subject of the sentence: it is the thing that begins.

So the structure is:

• Aðgerðin = the subject
• hefst = begins
• á morgun = tomorrow

A native English speaker can think of it as the Icelandic equivalent of The operation begins tomorrow, where the operation is the subject.

Hefst is the 3rd person singular present form of hefjast.

So:

• hefjast = to begin, to start
• hefst = begins / starts

It matches aðgerðin because aðgerðin is singular.

A useful thing to notice is that Icelandic present-tense verbs agree mainly with person and number, not with gender. So the verb is singular because the subject is singular.

The -st ending is very common in Icelandic and often appears in verbs that are:

• middle voice
• reflexive in origin
• or intransitive counterparts of other verbs

In this sentence, hefjast is the verb meaning to begin in the sense of to start on its own / to get underway.

Compare:

• hefja = to begin something, to start something
• hefjast = to begin, to start

So this sentence uses hefst because the operation itself is what begins.

Yes, absolutely. Byrjar is also a very common way to say begins / starts.

So both of these work:

• Aðgerðin hefst á morgun.
• Aðgerðin byrjar á morgun.

The version with hefst can sound a bit more formal or official in some contexts, while byrjar is often more everyday and conversational. But both are normal Icelandic.

Yes, word-for-word it looks like on tomorrow, but you should learn á morgun as a fixed Icelandic time expression meaning tomorrow.

This is one of those places where Icelandic and English simply organize time expressions differently.

A very useful contrast is:

• í morgun = this morning
• á morgun = tomorrow

That difference is important, because English speakers often mix them up at first.

Good question. The dictionary form is morgunn (morning), but in the fixed expression á morgun, you get the form morgun.

For learners, the easiest approach is to memorize á morgun as a whole expression meaning tomorrow.

You do not need to analyze it every time you use it. Just treat it like a standard time phrase, much the way you would memorize í dag for today.

Yes. Icelandic word order is fairly flexible, but the verb-second rule is important.

The neutral order here is:

• Aðgerðin hefst á morgun.

But you can also front the time expression:

• Á morgun hefst aðgerðin.

That still means the same thing: The operation begins tomorrow.

Notice what happens when á morgun comes first: the verb hefst still stays in the second position.

Not always. Aðgerð can mean different things depending on context, such as:

• operation (especially a medical operation)
• action
• measure
• procedure / intervention

So the exact English translation depends on the situation.

For example, in a medical context, aðgerðin is very likely the operation. In politics or administration, it might be the measure or the action instead.

It is completely normal Icelandic, but it sounds a bit neutral-to-formal, especially because of hefst.

You might see it in:

• news
• announcements
• official schedules
• hospital or administrative contexts

In very casual speech, many people might prefer byrjar:

• Aðgerðin byrjar á morgun.

So the sentence is natural, but slightly more formal in tone than the most conversational option.