Hun bruger en skuresvamp, når gryden er meget beskidt.

Because Danish usually makes nouns definite by adding the definite ending to the noun itself.

• gryde = pot
• gryden = the pot

Since gryde is an en-word (common gender), its singular definite form takes -en:

• en gryde = a pot
• gryden = the pot

This is different from English, which uses a separate word (the).

Because skuresvamp is a common-gender noun, so it takes en.

In Danish, singular nouns are either:

• common gender: take en
• neuter gender: take et

So:

• en skuresvamp = a scrubbing sponge

A useful clue is that Danish compound nouns usually take the gender of the last part. Here the last part is svamp (mushroom / sponge, depending on context), and svamp is an en-word, so skuresvamp is also an en-word.

Yes, it is one word, and that is very normal in Danish.

Danish often makes compound nouns by joining words together:

• skure = to scrub
• svamp = sponge
• skuresvamp = scrubbing sponge / scouring sponge

English sometimes writes similar ideas as two words or with a hyphen, but Danish usually prefers one combined word.

bruger is the present tense of bruge (to use).

So hun bruger can mean:

• she uses
• she is using

Which one sounds best in English depends on context. In this sentence, it most naturally describes a habitual or general action:

• She uses a scrubbing sponge when the pot is very dirty.

So it is closer to uses than is using here.

Because når gryden er meget beskidt is a subordinate clause.

The sentence has:

• main clause: Hun bruger en skuresvamp
• subordinate clause: når gryden er meget beskidt

The comma marks the boundary between them.

One important note: in modern Danish, comma use can vary slightly depending on whether someone uses start comma or not. So you may see both:

• Hun bruger en skuresvamp, når gryden er meget beskidt.
• Hun bruger en skuresvamp når gryden er meget beskidt.

Both can occur, but the version with the comma is very common and very clear for learners.

Because subordinate clauses in Danish do not use the main-clause verb-second pattern.

In a main clause, Danish often puts the finite verb in second position:

• Hun bruger en skuresvamp.

But after a subordinating word like når, the clause usually keeps the more straightforward order:

• når gryden er meget beskidt
• when the pot is very dirty

So the subject gryden comes before the verb er.

Because når is used for something that happens when/whenever a situation occurs.

Here the meaning is general or repeated:

• She uses a scrubbing sponge whenever the pot is very dirty.

Compare:

• når = when / whenever
  used for repeated or expected situations
• hvis = if
  more conditional
• da = when
  usually about a specific event in the past

So når is the natural choice here because the sentence describes a regular pattern, not a one-time past event and not a pure if-statement.

Because the adjective is being used predicatively after er (is), and the noun is singular.

Here:

• gryden = singular
• er beskidt = is dirty

In predicative position, Danish adjectives still show some agreement, but not in the same way as when they stand before a noun.

Compare:

• en beskidt gryde = a dirty pot
• gryden er beskidt = the pot is dirty
• beskidte gryder = dirty pots
• gryderne er beskidte = the pots are dirty

Also, the basic form of this adjective already ends in -t, so it may look unusual at first.

Here meget means very.

So:

• meget beskidt = very dirty

Although meget can also mean much/a lot, before an adjective it often works like very.

Why not mange?

• mange is used with plural countable nouns:
  mange gryder = many pots
• meget is used here as an adverb modifying an adjective:
  meget beskidt = very dirty

So mange beskidt would be wrong here.

In a normal sentence like this, Danish needs an explicit subject, so hun (she) must be there.

English and Danish are similar in this respect: you normally say

• She uses...
• Hun bruger...

You cannot normally just say Bruger en skuresvamp as a full standard sentence meaning She uses a scrubbing sponge. Danish usually requires the subject to be stated.

It most naturally describes a general habit or usual behavior.

That comes from the combination of:

• present tense bruger
• the clause with når (when/whenever)

So the sentence suggests something like:

• Whenever the pot is very dirty, she uses a scrubbing sponge.

It does not strongly sound like a single action happening right this second unless extra context makes that clear.