Om hun så kommer sent hjem, fylder hun stadig vandkanden og ser til kaninens bur.

Om has several meanings in Danish. In this sentence, it introduces a clause meaning if / even if.

Here, the pattern om ... så ... often gives a concessive meaning, so the sense is closer to even if than to simple if.

So:

• om = here, if / even if
• not about
• not the time meaning found in expressions like om en time = in an hour

A similar example would be:

• Om det så regner, går vi alligevel.
  = Even if it rains, we’re going anyway.

Here så is not really the ordinary so or then from English.

In the pattern om ... så ..., så helps strengthen the idea of concession: even if, even when, no matter that. It often does not need a separate word-for-word translation.

So:

• om hun kommer sent hjem = if she comes home late
• om hun så kommer sent hjem = even if she comes home late

It adds emphasis, as if saying: even in that case.

This is because Danish is a V2 language in main clauses. That means the finite verb normally comes in the second position.

The first position here is the whole clause:

• Om hun så kommer sent hjem

After that, the finite verb of the main clause must come next:

• fylder

Then comes the subject:

• hun

So the structure is:

• [Om hun så kommer sent hjem], fylder hun stadig vandkanden ...

If the sentence started directly with the subject, then you would get normal subject-verb order:

• Hun fylder stadig vandkanden ...

This inversion after a fronted element is very common in Danish.

Because om hun så kommer sent hjem is a subordinate clause, and subordinate clauses follow different word-order rules from main clauses.

The important thing for a learner is:

• In the subordinate clause, the subject hun comes before the finite verb kommer
• In the main clause, after a fronted element, the verb comes before the subject: fylder hun

So the sentence shows both patterns at once:

• subordinate clause: hun kommer
• main clause after fronting: fylder hun

That contrast is very typical in Danish.

Stadig means still here.

It shows that the action continues or happens despite the situation in the first clause. In natural English, you might translate it as:

• still
• nevertheless
• anyway

So the idea is:

• Even if she gets home late, she still fills the watering can and checks on the rabbit’s cage.

Without stadig, the sentence would still be grammatical, but the contrast would be weaker.

No. At se til is an expression, and it means something like:

• check on
• look after
• see to
• make sure something is okay

So ser til kaninens bur does not mean simply looks at the rabbit’s cage. It means more like checks on the rabbit’s cage or sees to the rabbit’s cage.

This is a good example of a verb + preposition combination that should be learned as a unit:

• se = see
• se til = check on / look after

Kaninens is the genitive form of kaninen.

Step by step:

• kanin = rabbit
• kaninen = the rabbit
• kaninens = the rabbit’s

Then bur means cage.

So:

• kaninens bur = the rabbit’s cage

A very important point: after the genitive, the next noun usually stays in its basic form.

So Danish says:

• kaninens bur
• not kaninens buret

This is just like:

• mandens bil = the man’s car
• husets tag = the house’s roof

Because vandkanden is the definite form: the watering can.

In Danish, the definite article is often added as an ending:

• en vandkande = a watering can
• vandkanden = the watering can

The sentence is referring to a specific watering can that is already known from the context, so the definite form is natural.

This is very common in Danish: instead of a separate word like English the, Danish often uses a noun ending.

Because hjem works as an adverb meaning homeward / home in the sense of movement toward home.

So:

• komme hjem = come home
• gå hjem = go home

No preposition is needed.

Also notice the difference between:

• hjem = home, as direction
• hjemme = at home, as location

So:

• Hun kommer hjem. = She comes home.
• Hun er hjemme. = She is at home.

In your sentence, sent simply means late, so kommer sent hjem = comes home late.

Yes, in many contexts selvom could be used to express a similar idea:

• Selvom hun kommer sent hjem, fylder hun stadig vandkanden ...

That would also mean although / even though she comes home late ...

But om ... så ... has a slightly different flavor. It often sounds more like:

• even if
• no matter whether
• even in that case

So the original sentence gives a stronger sense of regardless of that circumstance, she still does it.

Very roughly:

• selvom = although / even though
• om ... så ... = even if

In real usage, the line is not always sharp, but that is a good practical distinction for learners.