När gästerna kom, fick de varsin stol och varsitt glas vatten.

Gästerna is the definite plural form of gäst (guest).

• en gäst = a guest
• gäster = guests
• gästerna = the guests

So När gästerna kom means When the guests arrived/came. Swedish often uses the definite form where English would also use the.

Because the sentence is in the past tense.

• kom = past tense of komma (come / arrive)
• fick = past tense of få (get / receive)

So the sentence describes something that happened in the past:

• När gästerna kom = When the guests came/arrived
• fick de ... = they got / were given ...

This is because Swedish follows the V2 rule in main clauses: the finite verb usually comes in the second position.

Here, the sentence begins with a subordinate clause:

• När gästerna kom = When the guests arrived

After that whole clause, the main clause starts, and the finite verb must come first in the main clause:

• fick de varsin stol...

So the order is:

1. first element: När gästerna kom
2. finite verb: fick
3. subject: de

This is very common in Swedish:

• Igår köpte jag en bok.
• När jag kom hem, åt vi middag.

Not:

• När jag kom hem, vi åt middag.

Here de is the subject pronoun, meaning they.

• de = they (subject)
• dem = them (object)

In this sentence, de is the one doing the receiving in the main clause:

• fick de varsin stol... = they got one chair each...

So de is correct.

A note for spoken Swedish: many speakers pronounce both de and dem as dom, but in writing the distinction is usually kept.

Varsin means one each or one apiece.

It is used when several people each get one of something.

So:

• de fick varsin stol = they each got a chair

This is different from just saying:

• de fick en stol = they got a chair

That could mean one chair total, shared somehow, or it may simply not emphasize the distribution.
Varsin clearly tells you that each person got one.

Because varsin changes form to match the gender and number of the noun that follows.

Here are the basic forms:

• varsin with common gender (en-words)
• varsitt with neuter gender (ett-words)
• varsina with plural nouns

In the sentence:

• en stol → common gender → varsin stol
• ett glas → neuter gender → varsitt glas

So the form of varsin agrees with the noun, just like many other Swedish determiners and adjectives do.

In Swedish, ett glas vatten is the normal way to say a glass of water.

Swedish often puts these nouns directly next to each other:

• en kopp kaffe = a cup of coffee
• ett glas mjölk = a glass of milk
• en flaska vin = a bottle of wine

Using av here would sound unnatural in ordinary Swedish. So ett glas vatten is the standard expression.

Because vatten here is an uncountable substance noun, like water in English.

You say:

• ett glas vatten = a glass of water

not:

• ett glas ett vatten

Just like in English, you usually do not put a/an before water when it means the substance in general.

Yes, you could, but the meaning would be less precise.

• en stol och ett glas vatten = a chair and a glass of water
• varsin stol och varsitt glas vatten = a chair each and a glass of water each

The sentence with varsin / varsitt emphasizes individual distribution. It tells us that every guest got their own chair and their own glass of water.

Without varsin / varsitt, that idea is weaker or may need to be understood from context.

It is usually used when something is distributed among separate owners or recipients, often people or animals treated individually.

Examples:

• Barnen fick varsin glass. = The children got an ice cream each.
• Vi hade varsitt rum. = We had a room each.

So it is strongly connected to the idea of one per person/individual.

Both can work, depending on context.

• kom literally corresponds to came
• but in many situations, English naturally says arrived

So När gästerna kom can be understood as:

• When the guests came
• When the guests arrived

If the guests are arriving at a place, arrived is often the most natural English translation, even though the Swedish verb is simply kom.

The comma separates the introductory subordinate clause from the main clause:

• När gästerna kom, = subordinate clause
• fick de varsin stol... = main clause

In Swedish, commas after initial subordinate clauses are common and often helpful for readability. You may also see some variation in less formal writing, but here the comma is completely natural.

Yes. A natural alternative is:

• De fick varsin stol och varsitt glas vatten när gästerna kom.

But that version can sound slightly different in focus. The original starts with the time clause När gästerna kom, so it foregrounds when this happened.

The original sentence is especially natural if the arrival is the setup and the main point is what happened next.