Pottemakeren reparerer sprekken med en gang han ser den.

Because -en is the usual definite singular ending for many masculine/common nouns in Bokmål. So:

• en pottemaker = a potter
• pottemakeren = the potter

Unlike English, Norwegian often puts the onto the end of the noun instead of using a separate word.

Pottemaker means potter: someone who makes pottery, clay pots, ceramics, and similar things.

It is made up historically from parts meaning something like pot + maker, so it is fairly transparent once you know the vocabulary.

In Bokmål, many verbs in the present tense end in -er. The infinitive is å reparere = to repair, and the present tense is:

• jeg reparerer = I repair / am repairing
• han reparerer = he repairs / is repairing

So reparerer is simply the present tense form of å reparere.

It can mean either, depending on context.

Norwegian present tense often covers both:

• he repairs
• he is repairing

So Pottemakeren reparerer sprekken ... could describe:

• a habitual action, or
• an action happening in the present, or
• sometimes even a near-future action depending on context.

English usually has to choose more specifically; Norwegian often does not.

Sprekken is the definite form of sprekk, which means crack.

So:

• en sprekk = a crack
• sprekken = the crack

Again, the idea of the is added as a suffix.

Because Norwegian normally expresses definiteness by attaching the article to the noun.

Compare:

• English: the potter

• Norwegian: pottemakeren

• English: the crack
• Norwegian: sprekken

So the sentence does not need a separate standalone word like English the in these cases.

Here med en gang means immediately or as soon as.

Literally, it looks like with one time / with one go, but idiomatically it means something like:

• right away
• immediately
• as soon as

In this sentence, because it is followed by a clause (han ser den), the most natural English meaning is as soon as.

Not always. It can also mean immediately in a more general sense.

For example:

• Kom hit med en gang! = Come here immediately!

But in a sentence like this one:

• med en gang han ser den

it works like as soon as he sees it.

So the exact English translation depends on what comes after it.

Because han ser den is a subordinate clause after the time expression med en gang.

Inside that clause, Norwegian keeps the normal subject–verb order:

• han = subject
• ser = verb
• den = object

So:

• han ser den = he sees it

This is normal and expected after expressions like med en gang, når, fordi, hvis, and so on.

Han refers to pottemakeren.

So the sentence means that the potter repairs the crack as soon as he sees it.

Since pottemakeren refers to a male person, han is the natural pronoun.

Den refers to sprekken.

So:

• han ser den = he sees it
• it = the crack

Norwegian uses den for many nouns of common gender, and sprekk is treated that way here.

Because the pronoun matches the grammatical gender of the noun it refers to.

Here the noun is sprekk, which in this sentence is treated as a common-gender noun:

• en sprekk
• sprekken
• therefore den

If the noun were neuter, you would typically use det instead.

This is a very good Bokmål question.

Many nouns that are historically feminine can often be written either with feminine forms or with common-gender forms in Bokmål. So you may see:

• en sprekk / sprekken and sometimes
• ei sprekk / sprekka

In this sentence, the writer has chosen the common-gender style:

• sprekken
• pronoun den

That is completely normal in Bokmål.

In modern Norwegian, a comma is usually not required before a subordinate clause when the clause comes after the main clause.

So:

• Pottemakeren reparerer sprekken med en gang han ser den.

is perfectly standard without a comma.

Older comma habits and some learners’ expectations from English may make a comma seem possible, but standard Norwegian normally leaves it out here.

Yes. Så snart han ser den is a very natural alternative and means almost the same thing: as soon as he sees it.

Possible versions:

• Pottemakeren reparerer sprekken med en gang han ser den.
• Pottemakeren reparerer sprekken så snart han ser den.

The first can feel a bit more like immediately/right away, while the second is a very straightforward as soon as expression.

Because reparerer is a finite verb, not an infinitive.

• å reparere = to repair
• reparerer = repairs / is repairing

A full sentence needs a finite verb, and here that finite verb is reparerer.

Yes. The present tense allows that.

Depending on context, it could mean something like:

• The potter repairs the crack as soon as he sees it
  as a general practice or habit,

or it could describe a specific present situation.

Norwegian often leaves that distinction to context, whereas English sometimes makes it clearer with wording like whenever he sees it or is repairing it as soon as he sees it.