Jag måste sätta mig ner en stund.

A word-by-word breakdown:

• Jag = I
• måste = must / have to
• sätta = set / put
• mig = myself / me
• ner = down
• en stund = a while / a short time

But Swedish does not translate word-for-word here. The whole expression sätta sig ner means to sit down.

So the structure is roughly:

• Jag måste = I have to
• sätta mig ner = sit down
• en stund = for a while

Because sätta sig ner and sitta ner are not quite the same.

• sätta sig ner = to sit down
  This focuses on the movement: going from standing or being upright to a seated position.

• sitta = to sit / be sitting
  This describes the state of already being seated.

So:

• Jag måste sätta mig ner en stund = I need to sit down for a while
• Jag måste sitta en stund = I need to sit for a while

In many situations, both can be possible, but sätta sig ner often sounds more natural if the idea is that you need to lower yourself into a chair, sofa, etc.

Because sätta sig ner is a reflexive verb phrase in Swedish.

The basic pattern is:

• sätta sig ner = sit down
• literally something like set oneself down

The reflexive pronoun changes with the subject:

• jag sätter mig ner = I sit down
• du sätter dig ner = you sit down
• han/hon sätter sig ner = he/she sits down
• vi sätter oss ner = we sit down

So in your sentence, mig matches jag.

They are different reflexive pronouns.

• mig goes with jag
• dig goes with du
• sig goes with han, hon, den, det, and sometimes formal Ni
• oss goes with vi
• er goes with ni

Examples:

• Jag måste sätta mig ner. = I have to sit down.
• Hon måste sätta sig ner. = She has to sit down.

So you cannot say Jag måste sätta sig ner, because sig does not match jag.

Ner means down, and in sätta sig ner it helps form the common expression sit down.

It emphasizes direction or movement downward into a seated position.

Compare:

• sätta sig = can sometimes already imply sitting down
• sätta sig ner = very clearly sit down

In everyday Swedish, ner is very common in this phrase. It makes the meaning feel complete and natural.

Yes. Ner and ned are closely related forms, and both can mean down.

So both are possible:

• Jag måste sätta mig ner en stund.
• Jag måste sätta mig ned en stund.

In modern everyday spoken Swedish, ner is often more common and more conversational. Ned can feel a bit more formal or written in some contexts, though both are correct.

Because after modal verbs in Swedish, you normally use the infinitive without att.

Common modal verbs include:

• måste = must
• kan = can
• vill = want
• ska = shall / will / am going to
• får = may / get to

So you say:

• Jag måste sätta mig ner.
• Jag kan hjälpa dig.
• Hon vill gå hem.

Not:

• Jag måste att sätta mig ner.

That would be incorrect in standard Swedish.

Because Swedish main clauses usually follow the V2 rule: the finite verb comes in the second position.

In this sentence:

• Jag = first element
• måste = finite verb, so it comes second

Then the rest follows:

• sätta mig ner en stund

This is why the basic order is:

• Jag måste sätta mig ner en stund.

And if you move another element to the front, the verb still stays second:

• Nu måste jag sätta mig ner en stund.
• Efter jobbet måste jag sätta mig ner en stund.

That is a very important Swedish word-order pattern.

Because stund is a common-gender noun, so it takes en, not ett.

• en stund = a while
• stunden = the while / the moment

This is just something you have to learn with the noun.

Also, en stund is a very common fixed expression meaning:

• for a while
• for a bit
• for a short time

Examples:

• Vänta en stund. = Wait a moment.
• Jag vilar en stund. = I’ll rest for a while.

No. In Swedish, time expressions like this often appear without a preposition.

So:

• en stund = for a while
• en timme = for an hour
• hela dagen = all day

Examples:

• Jag sov en timme. = I slept for an hour.
• Hon väntade en stund. = She waited for a while.

So in your sentence, en stund works directly as a time expression without adding anything.

Yes, you may hear that, but it gives a slightly different feeling.

• Jag måste sätta mig ner en stund = I need to sit down for a while
  Focus on the action of sitting down.

• Jag måste sitta ner en stund = I need to be sitting for a while / I need to sit for a while
  Focus more on remaining seated.

In real speech, the difference is sometimes small, but sätta mig ner is the clearer choice if you mean the moment of sitting down.

Only the subject and reflexive pronoun need to change.

Examples:

• Du måste sätta dig ner en stund. = You have to sit down for a while.
• Han måste sätta sig ner en stund. = He has to sit down for a while.
• Hon måste sätta sig ner en stund. = She has to sit down for a while.
• Vi måste sätta oss ner en stund. = We have to sit down for a while.
• Ni måste sätta er ner en stund. = You all have to sit down for a while.
• De måste sätta sig ner en stund. = They have to sit down for a while.

Notice that måste does not change for person, unlike English must vs. other forms.

Yes, very natural.

A Swedish speaker might say this when they are:

• tired
• dizzy
• overwhelmed
• needing a short break

It sounds everyday and idiomatic.

Very similar natural alternatives include:

• Jag måste sätta mig ner lite. = I need to sit down a bit.
• Jag måste vila en stund. = I need to rest for a while.
• Jag behöver sätta mig ner en stund. = I need to sit down for a while.

So your sentence is fully normal and useful.