Det er to dage siden, hun var på skadestuen, og hævelsen ved anklen er allerede mindre.

This is a very common Danish way to talk about how much time has passed since something happened.

Structure:

• Det = a dummy subject, like it in It has been two days
• er = is
• to dage = two days
• siden = since

So Det er to dage siden, hun var på skadestuen corresponds to English It has been two days since she was at the ER.

Danish often uses this structure where English might also simply say She was at the ER two days ago.

Because the sentence connects:

• a present situation: two days have now passed
• with a past event: she was at the emergency room

So:

• er refers to the situation now
• var refers to what happened then

This is very similar to English:

• It has been two days since she was at the emergency room.

Here siden means since in a time sense.

In this pattern, it introduces the earlier event:

• Det er to dage siden, hun var på skadestuen
  = It is/has been two days since she was at the emergency room

A useful comparison:

• for to dage siden = two days ago
• det er to dage siden, hun var ... = it has been two days since she was ...

So siden is not just a simple vocabulary item here; it is part of a very common time expression.

In Danish, på is often used with institutions, departments, or places you go to for a purpose.

So:

• på skadestuen = at the emergency room / in the ER as a patient or visitor
• i skadestuen would sound more like inside the physical room/space

In this context, på skadestuen is the idiomatic choice.

You will see the same kind of pattern in other expressions, for example:

• på hospitalet
• på skolen
• på kontoret

Because Danish uses the definite form when the noun is specific and identifiable from the context.

Here:

• skadestuen = the emergency room relevant to the situation
• hævelsen = the swelling already known to the speaker/listener
• anklen = the ankle being discussed

Remember that in Danish, the definite article is usually attached to the end of the noun:

• en hævelse = a swelling
• hævelsen = the swelling

Also, Danish often uses the definite form with body parts where English might use a possessive:

• Danish may say something equivalent to the ankle
• English may prefer her ankle

Ved means something like by, near, or around.

So hævelsen ved anklen suggests:

• swelling at the ankle area
• not necessarily only on the surface of the ankle

Compare the nuance:

• ved anklen = around/by the ankle area
• på anklen = on the ankle
• i anklen = in the ankle, possibly suggesting inside the joint

In a medical or everyday description, ved anklen sounds natural and broad.

Because allerede is an adverb modifying the predicate mindre.

So:

• er allerede mindre = is already smaller / is already less swollen

This word order is very natural in Danish:

• verb: er
• adverb: allerede
• adjective/complement: mindre

You will often see this pattern:

• Han er allerede træt
• Det er allerede bedre
• Hævelsen er allerede mindre

Yes. Mindre is the comparative form connected to lille.

The forms are:

• lille = small
• mindre = smaller / less
• mindst = smallest / least

In this sentence, mindre works well because swelling is something that can become less or smaller.

So:

• hævelsen er allerede mindre
  = the swelling is already smaller / has already gone down some

In natural English, you might translate the idea more freely as:

• the swelling is already going down
• the swelling is already less severe

Because og is a coordinating conjunction, and it does not cause inversion.

After og, normal main-clause word order stays:

• subject + verb

So:

• og hævelsen ... er allerede mindre

This is different from cases where something else is placed first and triggers inversion, for example:

• I dag er hævelsen mindre
• Allerede nu er hævelsen mindre

But after og, you normally just continue with standard order:

• og hun kommer senere
• og hævelsen er allerede mindre

Not always. This depends on comma style.

You may see:

• Det er to dage siden, hun var på skadestuen
• Det er to dage siden hun var på skadestuen

Both can occur. The comma before a subordinate clause is often a style choice depending on whether someone uses the more traditional comma system.

The comma before og is more straightforward, because it separates two coordinated main clauses:

• Det er to dage siden ..., og hævelsen ...

So the first comma is more optional/style-based than the second one.

Yes, in many situations the meaning is very close, but the structure and emphasis are slightly different.

• Hun var på skadestuen for to dage siden
  = She was at the emergency room two days ago
• Det er to dage siden, hun var på skadestuen
  = It has been two days since she was at the emergency room

The second version emphasizes the time that has passed up to now. The first version simply places the event in the past.

So they are often interchangeable in meaning, but not identical in focus.

Danish often uses the simple present in this pattern:

• Det er to dage siden ...

English usually prefers:

• It has been two days since ...

So the Danish sentence is perfectly normal even though English uses a present perfect structure.

This is one of those places where a very literal word-for-word comparison is not helpful. The natural Danish expression is simply:

• Det er to dage siden ...

That is the standard way to say it.