Når en rod er blevet for lang, må hun skifte potte, så planten ikke stopper med at vokse.

Here når means when in the sense of a general situation or rule: whenever this happens, the next thing happens.

• når = when / whenever
• da = when for a specific event in the past
• hvis = if

So this sentence is presenting a general plant-care situation, not one single past event. That is why når fits best.

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

The sentence starts with the subordinate clause Når en rod er blevet for lang. Once that whole clause takes the first position, the main clause must begin with the finite verb:

• Når en rod er blevet for lang, må hun skifte potte ...

If there were no opening clause, you would normally say:

• Hun må skifte potte.

So the inversion is normal Danish word order.

Er blevet is the perfect tense of blive, which often means become.

So:

• blive = become
• er blevet = has become

That means er blevet for lang = has become too long.

This is a very common pattern in Danish:

• Det er blevet koldt. = It has become cold.
• Han er blevet træt. = He has become tired.

Because rod is a common-gender noun: en rod.

Predicate adjectives in Danish agree like this:

• common gender singular: lang
• neuter singular: langt
• plural / definite: lange

So:

• en rod er for lang
• et reb er for langt
• rødderne er for lange

That is why lang is correct here.

Normally, no. In standard Danish, hun usually refers to a female person, not to en rod or planten.

So out of context, this sentence sounds a bit odd. A native speaker would usually expect that a woman has already been mentioned earlier.

If the sentence is meant as a general instruction, more natural options would be:

• må man skifte potte
• skal du skifte potte
• må planten ompottes

So this is probably the part of the sentence an English-speaking learner should question most.

It can mean either in Danish, depending on context.

Here it most likely means must / has to / needs to, not may in the sense of permission. The context is about what is necessary in order to prevent a problem.

So in this sentence, må is best understood as a necessity word.

Compare:

• Du må gå nu. = You may go now. / You can go now.
• Jeg må gå nu. = I must go now.

Context decides the meaning.

Skifte potte is understandable, but in gardening Danish, a more idiomatic verb is often ompotte.

For example:

• Planten skal ompottes.
• Man må ompotte planten.

As for the missing article: Danish sometimes uses a bare noun in activity-type expressions, especially when the focus is on the action rather than on one specific object. Still, many speakers might prefer something more specific here, such as:

• skifte til en større potte
• skifte potten ud

So skifte potte is possible, but not the most natural wording in every context.

Here så means so that.

It introduces the purpose or result clause:

• så planten ikke stopper med at vokse
• so that the plant does not stop growing

This is not the then meaning of så. Danish så can have several meanings, and this is the so that type.

Because Danish normally uses stoppe med at + infinitive when it means stop doing something.

So:

• stoppe med at ryge = stop smoking
• stoppe med at tale = stop talking
• stoppe med at vokse = stop growing

That med at is the standard construction here.

Grammatically, en rod is fine if you really mean one root. But in plant-care advice, Danish often sounds more natural with rødderne or with the plant itself as the subject.

For example, a more natural sentence might be:

• Når rødderne er blevet for lange, må man potte planten om, så den ikke stopper med at vokse.

That sounds more like normal gardening advice and also avoids the strange hun.

The first comma is there because Når en rod er blevet for lang is a subordinate clause placed before the main clause.

So the comma marks the end of that opening clause:

• Når en rod er blevet for lang, må ...

The comma before så depends more on Danish comma style. Many writers include it, especially with a more traditional or clearer clause-marking style. In modern Danish, that comma may sometimes be omitted, depending on whether the writer uses start comma or not.

So the punctuation is not random; it reflects clause structure.