I forgårs snublede min søster på trappen og slog sin albue og sit håndled.

This is because Danish follows a verb-second pattern in main clauses.

When you put a time expression first, such as I forgårs (the day before yesterday), the finite verb must still stay in second position:

• I forgårs snublede min søster ...
• not I forgårs min søster snublede ...

So the structure is:

• I forgårs = time expression in first position
• snublede = finite verb in second position
• min søster = subject after the verb

This word order is very common in Danish.

I forgårs means the day before yesterday.

Here, i is part of a fixed time expression. You do not need to translate it word-for-word. Danish often uses i in time expressions:

• i dag = today
• i går = yesterday
• i morgen = tomorrow
• i forgårs = the day before yesterday

So it is best to learn i forgårs as a whole phrase.

Snublede is the past tense of snuble, meaning to trip or to stumble.

• infinitive: at snuble
• present: snubler
• past: snublede

So in this sentence, snublede shows that the action happened in the past.

Because slå is an irregular verb.

Its forms are:

• infinitive: at slå
• present: slår
• past: slog
• past participle: slået

So slog is simply the correct past tense form.

In this sentence, slog does not necessarily mean she intentionally hit something. It can also mean banged or hurt by impact, as in falling and hitting a body part.

In Danish, slå often literally means hit or strike, but with body parts it can also describe accidentally banging or injuring them.

So:

• slog sin albue = banged/hurt her elbow
• slog sit håndled = banged/hurt her wrist

This is very natural Danish. English often uses hurt where Danish uses slå.

Yes, trappen is grammatically singular, but Danish often uses the singular definite form for what English calls the stairs or the staircase.

• trappe = staircase / stairs
• trappen = the staircase / the stairs

So på trappen is a normal way to say on the stairs.

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

Here, the subject is min søster, and the elbow and wrist belong to her. So Danish normally uses sin/sit/sine rather than hendes.

• sin albue = her own elbow
• sit håndled = her own wrist

If you used hendes, it could suggest someone else’s elbow or wrist, not the subject’s own.

So:

• Min søster slog sin albue = My sister hurt her own elbow
• Min søster slog hendes albue = My sister hurt another female person’s elbow

Because sin and sit must agree with the gender of the noun they go with.

• albue is a common gender noun, so you use sin
• håndled is a neuter noun, so you use sit

The reflexive possessive forms are:

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

So:

• sin albue
• sit håndled
• sine hænder

Danish usually repeats the possessive when the nouns have different grammatical gender and therefore need different forms.

That is why you get:

• sin albue og sit håndled

You generally would not say:

• sin albue og håndled

because the second noun still needs its own possessive form, and it must be sit, not sin.

Because the subject min søster applies to both verbs:

• snublede
• slog

This is a standard coordination with og:

• min søster snublede og slog ...

In English we do the same thing:

• My sister tripped and hurt her elbow

We do not repeat my sister before the second verb unless we want special emphasis.

In a basic Danish main clause, yes, the subject often comes before the verb:

• Min søster snublede ...

But once another element is moved to the front, such as a time phrase, the verb stays second and the subject moves after it:

• I forgårs snublede min søster ...

So both are correct, but they have different word order because of the verb-second rule.

A more neutral subject-first version would be:

• Min søster snublede på trappen i forgårs og slog sin albue og sit håndled.

This means the same thing. The original sentence simply puts the time expression first for emphasis or flow.

Yes, this sentence is a great example of three very important Danish patterns:

1. Verb-second word order

   • I forgårs snublede min søster ...

2. Reflexive possessives

   • sin albue
   • sit håndled

3. Natural Danish vocabulary choices

   • snuble på trappen
   • slå a body part when you bump or injure it

If you understand those three things, you understand most of the grammar that makes this sentence feel specifically Danish.