Efter sådan et uheld er det vigtigt at hvile sig, selv hvis man kun har en lille hævelse.

Because uheld is a neuter noun in Danish, so it takes et, not en.

• en is used with common-gender nouns
• et is used with neuter nouns

So:

• et uheld = an accident / a mishap

With sådan, Danish often uses this pattern:

• sådan en bil = such a car
• sådan et uheld = such an accident

So et agrees with uheld.

Here sådan means such or that kind of.

So:

• sådan et uheld = such an accident
• more literally: that kind of accident

It does not mean so in the sense of very here.

Compare:

• sådan et uheld = such an accident
• så vigtigt = so important

In Danish, det is often used as a formal or dummy subject when the real subject is an infinitive clause.

Here, the real idea being talked about is:

• at hvile sig = to rest

But Danish usually says:

• Det er vigtigt at hvile sig = It is important to rest

This works just like English it in:

• It is important to rest

So det does not refer to a specific thing. It is just needed by the sentence structure.

Because hvile sig is the normal way to say to rest oneself / to rest in the sense of recovering.

• hvile can mean rest in some contexts, but often also lie or be at rest
• hvile sig is the common expression for a person taking rest

So:

• Jeg vil hvile mig = I want to rest
• Det er vigtigt at hvile sig = It is important to rest

This is a reflexive construction, which is very common in Danish.

Because sig is the reflexive pronoun used for the third person.

In this sentence, it refers back to man:

• man ... hvile sig

So the meaning is:

• one should rest
• a person should rest

You use sig when the subject and object are the same person in the third person.

Compare:

• Jeg hviler mig = I rest
• Du hviler dig = you rest
• Han hviler sig = he rests
• Man hviler sig = one rests

No. Here man means one, you, or people in general.

It is a very common Danish way to speak generally.

So:

• selv hvis man kun har en lille hævelse

means something like:

• even if you only have a small swelling
• even if one only has a small swelling

In natural English, you is often the best translation, but grammatically Danish uses man.

This is a very common question.

• selv hvis = even if
• selvom = even though / although

So selv hvis expresses a condition:

• selv hvis man kun har en lille hævelse = even if one only has a small swelling

This means the advice still applies under that condition.

If you used selvom, it would sound more like stating a fact:

• selvom man kun har en lille hævelse = even though one only has a small swelling

That is a different nuance. In your sentence, selv hvis is the right choice because it means even in the case that.

Because this part is a subordinate clause introduced by hvis.

In Danish subordinate clauses, words like kun, ikke, altid, ofte usually come before the finite verb.

So:

• hvis man kun har en lille hævelse = if one only has a small swelling

Compare the usual pattern:

• Main clause: Man har kun en lille hævelse
• Subordinate clause: ... hvis man kun har en lille hævelse

This is part of the normal Danish word order difference between main clauses and subordinate clauses.

Efter is a preposition, and here it means after.

• efter sådan et uheld = after such an accident

This whole prepositional phrase is placed at the beginning to set the context.

Danish often puts this kind of time or situation phrase first. When that happens, the verb still comes in the second position in the main clause:

• Efter sådan et uheld er det vigtigt ...

That is normal Danish main-clause word order.

Hævelse means swelling.

It comes from the idea of something being swollen or raised.

So:

• en hævelse = a swelling
• en lille hævelse = a small swelling

It is an en-word, which is why it takes:

• en
• lille

Because hævelse is a common-gender noun in Danish, so it takes en.

• en hævelse
• en lille hævelse

If it were a neuter noun, you would use et and often lille/lidt depending on the structure, but here the noun is simply an en-word.

So the phrase is just standard agreement:

• article: en
• adjective: lille
• noun: hævelse

Yes. Uheld can cover a range of meanings, from a mishap or unfortunate incident to an accident.

So depending on context, it might mean:

• accident
• mishap
• unfortunate event

In this sentence, because it is connected with resting and swelling, accident is a very natural translation. But the Danish word itself does not always have to mean a major accident.