Við skiptum um akrein þegar strætóinn stoppar fyrir framan okkur.

Because skipta um is a fixed expression meaning to change or to switch something.

In this sentence, skipta um akrein means to change lanes.
Without um, skipta often means things like divide, share, or exchange, so the meaning would change.

A good rule to remember is:

• skipta um föt = change clothes
• skipta um akrein = change lanes
• skipta um skoðun = change one’s opinion

Skiptum is the 1st person plural present tense form of skipta.

The subject is við, which means we, so the verb has to match that:

• ég skipti = I change
• þú skiptir = you change
• við skiptum = we change

So Við skiptum means we change.

Icelandic does not have an indefinite article like English a/an.

So akrein by itself can mean a lane.
If Icelandic wants to say the lane, it usually adds the definite article to the end of the noun:

• akrein = a lane / lane
• akreinin = the lane

That is why um akrein is perfectly normal for into a lane or change lanes, depending on context.

It is in the accusative singular, because skipta um normally takes the accusative.

So the structure is:

• skipta um + accusative

In this particular noun, the nominative and accusative singular look the same, so you do not see a visible ending change:

• nominative: akrein
• accusative: akrein

That is very common in Icelandic: the case is real, even when the form does not visibly change.

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

So:

• strætó = bus
• strætóinn = the bus

This ending -inn is the definite article attached to the noun.

Strætóinn means the bus, while strætó means a bus or just bus in a more general sense.

Using strætóinn tells us that this is a specific bus, probably one that the speaker and listener can identify from the situation.

So the sentence is talking about the bus in front of us, not just any bus.

Here þegar means when.

It introduces a time clause:

• þegar strætóinn stoppar = when the bus stops

A useful thing to know is that þegar can also mean already in other contexts.
So Icelandic learners often meet both meanings:

• Ég er þegar kominn = I have already arrived
• þegar strætóinn stoppar = when the bus stops

The clause after it tells you which meaning is intended.

Stoppar is the 3rd person singular present tense of stoppa.

It matches the subject strætóinn, which is singular:

• strætóinn stoppar = the bus stops

So the pattern is the same as in English: singular subject, singular verb form.

Fyrir framan okkur means in front of us.

It is best learned as a set expression:

• fyrir framan = in front of
• okkur = us

So:

• fyrir framan mig = in front of me
• fyrir framan þig = in front of you
• fyrir framan okkur = in front of us

Even though you can analyze the parts, learners usually do best by memorizing fyrir framan as a whole phrase.

Because Icelandic pronouns change form depending on their grammatical role.

• við = we as the subject
• okkur = us after a preposition or as a non-subject form

So in this sentence:

• Við skiptum = We change
• fyrir framan okkur = in front of us

This is similar to English we/us.

That is just a difference in how the two languages express the idea.

In English, change lanes is the usual idiom, even though you are really moving from one lane to another.
In Icelandic, skipta um akrein with singular akrein is completely normal.

So you should not force the English plural pattern onto Icelandic here.

Yes. Icelandic follows a verb-second pattern in main clauses.

So if the time clause comes first, the finite verb of the main clause usually comes next:

• Þegar strætóinn stoppar fyrir framan okkur, skiptum við um akrein.

Notice that it becomes skiptum við, not við skiptum.

That is a very important Icelandic word-order pattern.

Not if you want to say we change lanes.

The natural Icelandic expression is skipta um akrein.
Leaving out um would sound wrong here or suggest a different meaning.

So for this idea, treat skipta um as the verb unit you need.