Min søster går ind i omklædningsrummet, efter at hun har vist sit medlemskort ved receptionen.

Because each part adds something:

• går = goes / walks
• ind = in, inward
• i = into / in

So går ind i omklædningsrummet means goes into the changing room.

A useful contrast:

• Hun går ind = She goes inside.
• Hun går ind i omklædningsrummet = She goes into the changing room.

Danish often combines a direction word like ind with a preposition like i when talking about movement into a place.

Because Danish, like English to some extent, often makes compound nouns by joining words together, but Danish does this much more consistently.

omklædningsrummet can be broken down like this:

• omklædning = changing / getting changed
• -s- = a linking sound often used in compounds
• rum = room
• -met / -et = the definite ending here as part of rummet

So omklædningsrum = changing room, and omklædningsrummet = the changing room.

That ending is the definite article attached to the noun.

In Danish, instead of putting the before the noun, you often add the definiteness to the end:

• et omklædningsrum = a changing room
• omklædningsrummet = the changing room

The noun omklædningsrum is a neuter noun, so its definite singular ending is -et.

Compare:

• en reception = a reception
• receptionen = the reception

Because efter at hun har vist sit medlemskort ved receptionen is a subordinate clause.

In Danish, many writers place a comma before a subordinate clause. So this version is completely normal.

You may also see the sentence written without that comma:

Min søster går ind i omklædningsrummet efter at hun har vist sit medlemskort ved receptionen.

Both styles are accepted in modern Danish, as long as the writer is consistent.

Because after a subordinating conjunction like efter at, Danish uses subordinate-clause word order.

That means:

• conjunction: efter at
• subject: hun
• finite verb: har
• past participle: vist

So efter at hun har vist is correct.

This is different from the special verb-second pattern of main clauses in Danish. A very useful comparison is:

• Main clause: Hun har vist sit medlemskort.
• Subordinate clause: efter at hun har vist sit medlemskort

And if the subordinate clause comes first, the main clause shows inversion:

• Efter at hun har vist sit medlemskort ved receptionen, går min søster ind i omklædningsrummet.

har vist is the present perfect:

• har = has
• vist = shown

So literally it is has shown.

It is used here because the showing of the membership card is completed before the next action, which is entering the changing room.

So the sequence is:

1. she shows the card
2. she goes into the changing room

Danish often uses the present perfect very naturally to show that one action has already been completed before another.

Because Danish uses a reflexive possessive when the possessor is the subject of the clause.

Here the subject is hun, and the card belongs to her, so Danish uses reflexive sin/sit/sine.

Since medlemskort is a neuter singular noun, the correct form is sit:

• sin for common-gender singular nouns
• sit for neuter singular nouns
• sine for plural nouns

So:

• hun har vist sit medlemskort = she has shown her own membership card

If you said hendes medlemskort, it would normally point to someone else’s card, not the subject’s own card, or it would at least sound non-reflexive.

Because the possessive already makes the noun specific.

In Danish, after a possessive word like min, din, sin, sit, vores, and so on, the noun normally stays in the indefinite form:

• mit hus = my house
• sin jakke = his/her own jacket
• sit medlemskort = his/her own membership card

So you do not say sit medlemskortet.

This is similar to English: you say her card, not her the card.

ved receptionen means at the reception or by the reception desk.

The preposition ved is often used for being at or by a specific point, counter, desk, entrance, wall, window, and so on.

Here it suggests the place where she shows the card: the reception desk area.

A rough contrast:

• ved receptionen = at/by the reception desk
• i receptionen = in the reception area
• på receptionen = less natural here in standard usage

Also notice receptionen is definite, because it refers to the specific reception in that building.

Not in this exact structure.

Use:

• efter at
  • a full clause
    
    • efter at hun har vist sit medlemskort

But use:

• efter
  • a noun or noun phrase
    
    • efter træningen = after the training session
    • efter besøget = after the visit

So in this sentence, because you have a full clause with subject and verb, efter at is the normal choice.

min søster is the neutral, standard way to say my sister.

Danish usually places possessives before the noun:

• min søster
• mit kort
• vores bil

You may sometimes hear forms like søsteren min in speech, and they can sound more colloquial, expressive, or emphatic. But for a learner, min søster is the safest and most standard choice.

vist is the past participle of vise, which means to show.

The forms are:

• vise = to show
• viser = shows / is showing
• viste = showed
• vist = shown

So in har vist, the word vist works with har to form the present perfect:

• hun har vist sit medlemskort = she has shown her membership card

So it is not the simple past by itself; it is the participle used in a compound tense.