Hun siger, at det er længe siden, hun sidst havde et uheld, og at hun nu går langsommere på trappen.

Because there are two subordinate clauses after Hun siger:

• at det er længe siden, hun sidst havde et uheld
• og at hun nu går langsommere på trappen

Both clauses depend on Hun siger. In English, you can sometimes omit the second that, but in Danish, repeating at is very common and makes the structure clearer.

Det er længe siden is a very common Danish expression meaning it has been a long time or it’s a long time since.

The det here is an impersonal subject, like English it in:

• It is raining
• It is a long time since...

It does not refer to a specific thing. It is just part of the normal structure.

After det er længe siden, Danish often goes straight into the clause:

• Det er længe siden, hun sidst havde et uheld

This is a normal pattern. The part after the comma explains since when or since what last happened.

So the structure is similar to English:

• It’s been a long time since she last had an accident

You do not need another at there.

Because Danish often uses the simple past in this kind of sentence when referring to the last time something happened:

• hun sidst havde et uheld

The focus is on a specific earlier event: the last accident she had.

This does not always match English tense usage exactly. English learners often expect a perfect tense, but Danish frequently uses the past tense after expressions like det er længe siden when talking about the earlier event itself.

Sidst means last or most recently.

So:

• hun sidst havde et uheld = the last time she had an accident

It makes the sentence more precise by pointing to the most recent occurrence. Without sidst, the sentence would still make sense, but sidst clearly emphasizes that we are talking about her last accident, not just any accident in the past.

Uheld is a neuter noun, so it takes et in the singular indefinite form:

• et uheld = an accident / a mishap

Some useful forms are:

• et uheld = an accident
• uheldet = the accident
• uheld = accidents

Also, uheld often suggests a mishap or unfortunate accident, and it can sound a bit less dramatic than ulykke, which is often used for a more serious accident.

Langsommere is the comparative form, meaning slower.

It comes from:

• langsom = slow
• langsomt = slowly / slow
• langsommere = slower / more slowly

In this sentence, it is used with the verb går:

• hun går langsommere = she walks more slowly / she walks slower

The comparison is usually understood as slower than before or slower than she used to.

Because på trappen is the usual Danish way to say on the stairs or on the staircase.

For movement or position involving stairs, Danish normally uses på:

• gå på trappen
• falde på trappen

Using i trappen would usually sound less natural here and might suggest being in the stairwell area rather than simply on the stairs.

Because Danish often treats a staircase as one unit:

• trappen = the staircase / the stairs

English very often uses a plural form, the stairs, but Danish commonly uses the singular definite:

• på trappen = on the stairs

So this is a case where Danish and English do not match word-for-word.

Because this is a subordinate clause, and Danish word order changes in subordinate clauses.

In a main clause, Danish often follows the V2 rule:

• Nu går hun langsommere på trappen

Here the adverb Nu comes first, so the verb går comes second.

But after at, you are in a subordinate clause, and the word order is more straightforward:

• at hun nu går langsommere på trappen

So the subject hun comes before the verb går.

The commas are marking clause boundaries.

The sentence contains several parts:

• Hun siger
• at det er længe siden, hun sidst havde et uheld
• og at hun nu går langsommere på trappen

The commas help show where one clause ends and another begins. In the version you were given, the punctuation makes the structure especially clear for the reader.

So even if the sentence is long, the commas show that it is built out of smaller pieces.