Breytingin á áætluninni kemur mér á óvart.

Because -in is the suffixed definite article.

• breyting = a change
• breytingin = the change

In Icelandic, the definite article is usually attached to the end of the noun rather than written as a separate word like English the.

Here, Breytingin is the subject of the sentence and means the change.

Breytingin is nominative singular.

It is nominative because it is the subject of the sentence — the thing that is doing the verb kemur.

So the basic structure is:

• Breytingin = the subject
• kemur = comes / surprises
• mér á óvart = me by surprise

This is a very common pattern: breyting á einhverju.

It means a change in/to something.

So:

• breyting á áætluninni = the change in the plan or the change to the plan

Here, á áætluninni modifies breytingin and tells you what the change is about.

Because áætluninni is dative singular definite.

The noun is:

• áætlun = plan

In this sentence, it follows á in the expression breyting á einhverju, which takes the dative.

So:

• áætlun = plan
• áætluninni = the plan, in the dative singular

That ending -inni is a very typical feminine dative singular definite ending.

Because the idiom koma einhverjum á óvart takes the person in the dative.

So:

• ég = I
• mig = me (accusative)
• mér = me (dative)

Since Icelandic says something comes to someone as a surprise, the someone is in the dative:

• kemur mér á óvart = surprises me / comes as a surprise to me

This is something you mostly have to learn as part of the whole expression.

It is a fixed expression meaning:

• to surprise someone
• to come as a surprise to someone

So in this sentence:

• Breytingin á áætluninni kemur mér á óvart
  = The change in/to the plan surprises me
  = The change in/to the plan comes as a surprise to me

It is best to learn koma á óvart as a chunk, not word by word.

Because they belong to two different parts of the sentence:

1. á áætluninni
   This goes with breyting:
   breyting á einhverju = a change in/to something

2. á óvart
   This goes with kemur mér in the idiom:
   koma einhverjum á óvart = to surprise someone

So even though both are á, they are doing different jobs.

Not always in every situation, but in this sentence, yes.

The preposition á can take different cases depending on meaning. But here:

• in breyting á einhverju, it takes the dative
• in the fixed expression koma einhverjum á óvart, you should just learn the phrase as it is

So this sentence has:

• á áætluninni with dative
• mér in dative because of the idiom

Yes, kemur is the 3rd person singular present of koma = to come.

So literally, the sentence is something like:

• The change to the plan comes to me as a surprise

But in natural English, you would usually say:

• The change to the plan surprises me

This is a good example of how Icelandic often uses an idiom where English uses a simpler verb.

Yes. Both can work, depending on context.

breyting á X often corresponds to English:

• a change in X
• a change to X

In this sentence, the change to the plan is often the most natural English choice, because it sounds like the plan was altered.

Yes, Icelandic word order is somewhat flexible, but the finite verb usually stays in the second position in main clauses.

So this sentence could also appear as:

• Mér kemur breytingin á áætluninni á óvart.

That still means the same thing, but it puts more emphasis on mér.

The version you were given is very normal and straightforward:

• subject first
• then verb
• then the rest

Here are the main ones:

• breyting = change
• áætlun = plan
• koma = come
• ég = I
• óvart / á óvart = by surprise, unexpectedly; in the phrase koma á óvart = surprise

This is often a useful way to study the sentence, because several words appear in inflected forms:

• Breytingin → from breyting
• áætluninni → from áætlun
• kemur → from koma
• mér → from ég