Þegar gestirnir koma, set ég kexið á bakka og helli gosinu í glös.

This is because Icelandic is a V2 language: in a main clause, the finite verb normally comes in the second position.

Here, the whole clause Þegar gestirnir koma takes the first position, so the verb set comes next:

• Þegar gestirnir koma, set ég ...
• literally: When the guests come, put I ...

If you started directly with the subject, you would get the more familiar order:

• Ég set kexið á bakka og helli gosinu í glös.

So set ég is completely normal here.

In Icelandic, the definite article is usually attached to the end of the noun, not written as a separate word.

In this sentence:

• gestirnir = the guests
• kexið = the biscuits/cookies/crackers
• gosinu = the soda / the soft drink

By contrast, these nouns are indefinite:

• á bakka = on/onto a tray
• í glös = into glasses

So Icelandic often expresses the as a noun ending rather than a separate word.

Because the subject is gestirnir, which is plural: the guests.

So the verb must agree with that plural subject:

• gestirnir koma = the guests come/arrive

Compare:

• gesturinn kemur = the guest comes
• gestirnir koma = the guests come

So koma is the correct present-tense plural form here.

It can mean either, depending on context.

The present tense in Icelandic, just like in English, can be used in time clauses for:

• a future event: When the guests come, I’ll put...
• a habitual action: Whenever the guests come, I put...

This sentence sounds very natural as a routine/habitual statement, but it can also be understood as a future arrangement depending on the situation.

Because kex in Icelandic is often used like a collective or mass noun.

So kexið can mean something like:

• the biscuits
• the cookies
• the crackers
• the snack biscuits

In other words, Icelandic does not always count these the same way English does. Using the singular form here does not necessarily mean there is only one piece.

Because the verb hella usually takes the thing being poured in the dative case.

So:

• hella gosinu í glös = pour the soda into glasses

Here:

• gosið would be nominative/accusative definite
• gosinu is dative definite

This is one of those verbs where you simply need to learn which case it governs.

A helpful way to remember it is:

• hella einhverju í eitthvað
• pour something-DAT into something-ACC

Because after í, Icelandic uses:

• accusative for motion/destination
• dative for location

Here, the soda is being poured into the glasses, so there is movement toward the inside of them. That is why you get:

• í glös = into glasses (accusative)

Compare:

• Gosið er í glösum. = The soda is in glasses. (location, so dative)

So this is the classic Icelandic motion vs. location distinction.

The same motion vs. location rule applies with á:

• accusative = movement onto
• dative = position on

Since the sentence means putting the biscuits onto a tray, the underlying case is accusative.

However, there is a small complication: with the noun bakki, the singular form bakka looks the same in both accusative and dative. So from the form alone, you cannot see the difference here.

A clearer comparison is with the definite forms:

• á bakkann = onto the tray
• á bakkanum = on the tray

So in your sentence, á bakka fits the meaning onto a tray, even though the form itself is not visibly different.

Because the same subject continues across both verbs.

So:

• set ég kexið á bakka og helli gosinu í glös
• means I put the biscuits on a tray and pour the soda into glasses

Icelandic, like English, often leaves the subject unstated in the second part when it is clearly the same person.

You could repeat it for emphasis, but normally you do not need to:

• ... og ég helli gosinu í glös

That is possible, but less neutral here.

Yes, that comma is natural and standard.

The first part:

• Þegar gestirnir koma

is a subordinate clause, and Icelandic commonly puts a comma after an initial subordinate clause before the main clause:

• Þegar gestirnir koma, set ég ...

So the punctuation here is correct and helpful.

A few useful pronunciation points:

• Þ in Þegar is like th in thing
• ð does not appear in this sentence, but learners often confuse it with þ; ð is like th in this
• j is pronounced like English y
  • so ég sounds roughly like yeg
• ll in helli is not a normal English ll sound; many learners can start with an approximation like hetli
• gestirnir has the -nir ending attached to the noun; say it as one word, not as a separate the

A rough learner-friendly approximation of the whole sentence might be:

• THEH-gar GES-tir-nir KO-ma, set yehg KEX-ith ow BAHK-ka og HET-li GO-si-nu ee GLEHS

It will not be perfect, but it is a useful starting point.