Hon satte upp en krok ovanför dörren, och sedan ställde hon tillbaka stegen i förrådet.

Satte upp is the past tense of the particle verb sätta upp.

In this sentence, sätta upp en krok means to put up / install / attach a hook. It does not just mean set in a general sense. The upp is part of the verb and helps create the meaning of mounting or putting something up on a wall or similar surface.

So:

• sätta = put, set
• sätta upp = put up, mount, install

Because the sentence is in the past tense.

The infinitive is sätta = to put / to set.
The past tense is satte = put / set.

Similarly:

• ställa → ställde
• sätta → satte

So the sentence is describing completed actions in the past:

• Hon satte upp... = she put up...
• hon ställde tillbaka... = she put back...

This is a very common question, because Swedish often uses different placement verbs depending on how something is positioned.

A simple way to think about it:

• sätta = set/put, often placing something into position or attaching it
• ställa = put something so that it stands upright
• lägga = put something so that it lies flat

In this sentence:

• satte upp en krok: a hook is being attached/mounted
• ställde ... stegen: a ladder is something that typically stands, so ställa fits naturally

This is one of those areas where Swedish is more specific than English.

Because sätta upp is a particle verb, and in Swedish these are often split in main clauses.

Compare:

• infinitive: att sätta upp en krok
• past tense main clause: hon satte upp en krok

The same thing happens with ställa tillbaka:

• infinitive: att ställa tillbaka stegen
• main clause: hon ställde tillbaka stegen

So the verb and particle belong together in meaning, even though they appear as separate words in the sentence.

Because this is a main clause, and Swedish main clauses usually follow the V2 rule: the finite verb comes in the second position.

Here, sedan comes first, so the verb must come next:

• sedan ställde hon tillbaka stegen

Structure:

• sedan = first position
• ställde = second position
• hon = subject after the verb

If you said sedan hon ställde, that would sound like the start of a subordinate clause, meaning something more like after she put..., not then she put....

Tillbaka means back.

So:

• ställde hon stegen i förrådet = she put the ladder in the storage room
• ställde hon tillbaka stegen i förrådet = she put the ladder back in the storage room

It shows that the ladder was returned to its previous or proper place.

The combination ställa tillbaka is very common and often means put back or return.

Both ovanför and över can relate to the idea of above, but they are not always interchangeable.

• ovanför usually means above, at a higher position than
• över is broader and can also mean over, across, above

In this sentence, ovanför dörren is the most natural way to say above the door as a location.

So ovanför is especially good when you are simply describing where something is positioned in space.

Because Swedish is making the same kind of distinction as English between a and the.

• en krok = a hook
  This is being introduced as a new item.

• dörren = the door
• stegen = the ladder
• förrådet = the storage room

These are treated as specific, identifiable things.

A useful thing to notice is that Swedish often puts the definite article at the end of the noun:

• dörr = door → dörren = the door
• stege = ladder → stegen = the ladder
• förråd = storage room → förrådet = the storage room

Yes, in this sentence it does.

The noun stege means ladder, and its definite singular form is stegen = the ladder.

This can be confusing because stegen can also be a form related to steg (step), depending on context. But here the context makes ladder clearly correct:

• you can ställa tillbaka stegen i förrådet = put the ladder back in the storage room

That fits naturally with a ladder, not with steps.

Because the sentence is talking about where the ladder ended up: in the storage room.

• i förrådet = in the storage room
• till förrådet = to the storage room

With a placement verb like ställa, Swedish often uses i when the focus is the final position inside a place.

So:

• Hon ställde stegen i förrådet = she placed the ladder in the storage room

If you wanted to emphasize the movement into the room, you could also hear something like in i förrådet, but here i förrådet is perfectly natural.

Because the second part is a new coordinated clause, and Swedish often states the subject again for clarity.

So you get:

• Hon satte upp en krok ovanför dörren, och sedan ställde hon tillbaka stegen i förrådet.

This sounds very natural, especially because sedan has been placed first in the second clause.

A more compressed version is also possible, for example:

• Hon satte upp en krok ovanför dörren och ställde sedan tillbaka stegen i förrådet.

Both are natural, but the original version is a little more explicit and neatly separates the two actions.