Selv efter et kort brusebad bruger min søster altid tandtråd om morgenen.

Because Danish main clauses normally follow the verb-second rule.

Here, the opening phrase Selv efter et kort brusebad takes the first position in the sentence. That means the finite verb bruger must come next, and the subject min søster comes after it.

So:

• Min søster bruger altid tandtråd om morgenen. = neutral word order
• Selv efter et kort brusebad bruger min søster altid tandtråd om morgenen. = the opening phrase is moved to the front for emphasis

This is a very common Danish pattern.

Here, selv means even, not self.

So Selv efter et kort brusebad means Even after a short shower.

Compare:

• selv = even
• sig selv = himself/herself/itself

In this sentence, selv adds a sense of surprise or emphasis.

Because efter is a preposition, while selvom is a conjunction.

• selv efter + noun phrase
  Selv efter et kort brusebad ...
  = Even after a short shower ...

• selvom + clause
  Selvom hun lige har taget et kort brusebad ...
  = Even though she has just taken a short shower ...

So in your sentence, et kort brusebad is a noun phrase, which is why efter is used.

Because brusebad is a neuter noun in Danish.

Its basic form is:

• et brusebad = a shower

When a singular indefinite neuter noun has an adjective before it, the adjective usually gets -t:

• et kort brusebad = a short shower

Compare:

• en kort tur = a short trip
• et kort brusebad = a short shower

So both et and kort come from the fact that brusebad is neuter.

Yes.

• brusebad specifically means shower
• bad is more general and can mean bath, bathing, or sometimes a wash/bathe

So et kort brusebad is specifically a short shower, not just any kind of bath.

Yes. In Danish, the natural expression is bruge tandtråd, literally use dental floss.

So:

• at bruge tandtråd = to floss

English has a special verb, to floss, but Danish often uses bruge + noun instead.

You may also hear longer expressions, but bruge tandtråd is the normal everyday way to say it.

Because tandtråd is being used as a mass noun here, just like English floss.

So Danish says:

• bruge tandtråd = use floss

not necessarily:

• bruge en tandtråd

That would sound like use one floss or refer to a countable item, which is not the usual meaning here.

If you wanted to refer to some specific floss, you might say something like:

• noget tandtråd = some floss
• tandtråden = the floss

But in the general activity, no article is needed.

Altid is an adverb of frequency, meaning always. In a normal Danish sentence, adverbs like this often come before the object.

So the core order here is:

• bruger = verb
• min søster = subject
• altid = adverb
• tandtråd = object

That makes bruger min søster altid tandtråd a very natural order.

The final phrase om morgenen is a time expression, and time expressions often come later in the sentence.

Om morgenen means in the morning or in the mornings, depending on context.

In this sentence, because of altid, it clearly means something habitual:

• altid ... om morgenen = always ... in the morning / every morning

By contrast:

• i morges = this morning (one specific morning, in the past)

So:

• Hun brugte tandtråd i morges. = She flossed this morning.
• Hun bruger altid tandtråd om morgenen. = She always flosses in the morning.

Because min is a possessive determiner, and it already tells you whose sister it is.

So:

• min søster = my sister
• søsteren = the sister

In standard Danish, you normally say min søster, not den min søster or just søsteren if you mean my sister.

So the sentence uses the normal Danish possessive pattern.

Yes. That is a completely natural sentence.

The difference is mostly emphasis:

• Min søster bruger altid tandtråd om morgenen.
  = straightforward statement

• Selv efter et kort brusebad bruger min søster altid tandtråd om morgenen.
  = puts emphasis on even after a short shower

So the version with Selv efter et kort brusebad sounds a bit more expressive or contrastive.