När potatisen är skalad kokar jag den med lök.

När means when.

In this sentence, it introduces a subordinate clause: När potatisen är skalad = When the potato is peeled / Once the potato has been peeled.

Depending on context, när can mean:

• when in a general sense
• once/after when one thing happens before the next

So here it gives the condition or timing for the main action: first the potato is peeled, then I boil it with onion.

Because potatisen is the definite form of potatis.

• potatis = potato / potatoes / potato as a substance
• potatisen = the potato

Swedish often uses the definite form where English also uses the. Instead of putting a separate word before the noun, Swedish usually adds the definite ending to the noun itself.

Here:

• potatis is an en-word
• so the definite singular form is potatisen

This is a very common thing in Swedish. A singular noun can be used in a collective or generic way, especially with food or ingredients.

So potatisen here can refer to:

• the potato as an ingredient in general
• the batch of potato being prepared

English often prefers the potatoes in similar situations, but Swedish may use singular where English would naturally use plural.

So even if more than one potato is meant in practice, potatisen can still sound natural in Swedish.

är skalad literally means is peeled.

This is formed with:

• är = is
• skalad = peeled

It describes the state of the potato after peeling. In other words, the sentence focuses on the result: the potato is in a peeled state.

That is a little different from saying:

• När jag har skalat potatisen ... = When I have peeled the potato ...

That version focuses more on the action done by me.
The original sentence focuses more on the potato being ready.

Because skalad agrees with potatisen, which is:

• singular
• common gender (en-word)

With a past participle used like an adjective after är, Swedish often shows agreement.

Here:

• en potatis → potatisen är skalad

Compare:

• ett äpple är skalat
• potatisarna är skalade

So:

• skalad = singular common gender
• skalat = singular neuter
• skalade = plural

Because Swedish follows the V2 rule in main clauses.

That means the finite verb must come in the second position.

The sentence starts with the subordinate clause:

• När potatisen är skalad

After that comes the main clause:

• kokar jag den med lök

Since the first position is already occupied by the whole När ... clause, the finite verb of the main clause must come next:

• kokar
• then jag

So Swedish says:

• När potatisen är skalad kokar jag den med lök.

not:

• När potatisen är skalad jag kokar den med lök.

Den refers back to potatisen.

Since potatis is an en-word, the matching pronoun is den.

Compare:

• en potatis → den
• ett äpple → det

So:

• potatisen = the potato
• den = it

Even though English might sometimes say them if several potatoes are meant, Swedish can still use singular den if the noun is being treated collectively or as one ingredient.

It means with onion or with onions, depending on context.

In Swedish, ingredient nouns are often used without making number as explicit as English sometimes does. So lök here can mean:

• onion as an ingredient in general
• some onion
• onions

The exact amount is not the main point. The phrase simply tells you what the potato is boiled with.

It is in the present tense:

• är = is
• kokar = boil / am boiling

In Swedish, the present tense is often used for:

• habitual actions
• instructions
• recipes
• general procedures

So the sentence can sound like:

• something I usually do
• a recipe-style description
• a step in a process

It does not have to mean the action is happening right now at this exact moment.

Usually, no.

In modern Swedish, a comma before the main clause after an initial subordinate clause is often left out:

• När potatisen är skalad kokar jag den med lök.

A comma is not impossible in some writing styles, but in normal modern Swedish it is very common to write the sentence without one.

Yes, that is also possible, but it is slightly different in focus.

• När potatisen är skalad ... focuses on the state/result: the potato is peeled.
• När jag har skalat potatisen ... focuses on my completed action: I have peeled the potato.

Both can be natural, but the original sentence sounds a bit more like a general procedure or recipe step, where the important thing is that the potato is ready for the next stage.

It can work in everyday speech, but it has a slightly procedural feel.

Because it says:

• När potatisen är skalad ...
• kokar jag den med lök

it sounds a bit like:

• explaining how you cook something
• describing your routine in the kitchen
• giving steps in order

In casual speech, some speakers might also say something more explicit, such as:

• När jag har skalat potatisen kokar jag den med lök

But the original sentence is still perfectly natural Swedish.