Learn Hardcore Swedish: Barnen måste passa sig vid sjön. - The children must be careful by the lake.

In the sentence, we are talking about specific, known children (for example, our children, the children we see), so Swedish uses the definite plural form barnen.

What exactly does måste do here, and why isn’t there att before passa?

Måste is a modal verb meaning something like must / have to.

In Swedish, modal verbs (like kan, vill, ska, måste) are followed directly by the infinitive form of the main verb without att:

Barnen måste passa sig – The children must be careful
Jag måste gå – I have to go
Hon ska äta – She is going to eat

So you say måste passa, not måste att passa and not måste passar. The modal verb itself (måste) doesn’t change with person: jag/du/han/vi/de måste are all the same form.

What does passa sig literally mean, and how is it different from just passa?

Passa sig is a reflexive verb that means something like watch out, mind yourself, be careful.

passa alone has several meanings:
- passa barn – to look after / babysit children
- Det passar bra – It suits/fits/is suitable
- passa bollen – to pass the ball (in sports)

When you add the reflexive pronoun sig, the meaning changes to a warning or caution:

Passa dig! – Watch out! / Be careful!
Barnen måste passa sig vid sjön. – The children must be careful at/by the lake.

So passa sig is not about fitting or babysitting; it is specifically about being careful, watching out for danger, etc.

What does sig refer to here, and does it change with plural/singular?

Sig is the third-person reflexive pronoun. It refers back to the subject barnen:

Barnen måste passa sig – The children must watch out (for themselves).

In Swedish, sig is used for both singular and plural in the 3rd person:

Han måste passa sig. – He must be careful.
Hon måste passa sig. – She must be careful.
De måste passa sig. – They must be careful.

For 1st and 2nd person, you use different pronouns:

Jag måste passa mig. – I must be careful.
Du måste passa dig. – You must be careful.
Vi måste passa oss. – We must be careful.
Ni måste passa er. – You (plural/formal) must be careful.

Can you leave out sig and just say Barnen måste passa vid sjön?

No, that would change (and basically break) the meaning.

Barnen måste passa sig is a fixed reflexive expression: the children must be careful / watch out.

If you say Barnen måste passa vid sjön, Swedish speakers will expect a different meaning of passa, like:

The children must take turns / be on duty at the lake.
The children must be suitable/fit at the lake. (which makes little sense)

So sig is necessary here to express the idea of being careful.

Why is it vid sjön and not i sjön or på sjön?

The preposition changes the spatial meaning:

vid sjön – by/at the lake, near the shore, not in the water
i sjön – in the lake, actually in the water
på sjön – literally on the lake, usually meaning out on the lake in a boat (like “out at sea”)

Since we’re talking about children being near the water and needing to be careful, vid sjön (by the lake) is the natural choice.

Why is it sjön (definite) and not just sjö?

Swedish uses the definite form a lot when English uses the:

en sjö – a lake
sjön – the lake

Vid sjön suggests a specific lake that is known from context (for example, the local lake everyone knows about, or the one just mentioned). If you said vid en sjö, it would mean by a lake (some random / unspecified lake).

In many fixed expressions with places, Swedish likes the definite form:

i skolan – at school
på landet – in the countryside
vid sjön – by the lake

Can the word order change, like Barnen måste vid sjön passa sig?

Normal, natural word orders would be:

Barnen måste passa sig vid sjön. (neutral, standard)
Vid sjön måste barnen passa sig. (emphasises by the lake as the setting)

Barnen måste vid sjön passa sig sounds awkward and unnatural. In Swedish, place adverbials like vid sjön often go at the end of the sentence, or they can be placed at the beginning for emphasis. Placing vid sjön in the middle here is not typical.

How is passa sig different from akta sig or vara försiktig?

They all deal with being careful, but with slightly different nuances:

passa sig – watch out, be careful, can sound a bit colloquial or like a warning:
- Du får passa dig! – You’d better watch yourself!
akta sig – very close in meaning, often used with för:
- Akta dig! – Watch out!
- Akta dig för bilen! – Watch out for the car!
vara försiktig – more neutral, literally be careful/cautious:
- Barnen måste vara försiktiga vid sjön.

In your sentence, Barnen måste passa sig vid sjön feels like a more vivid, slightly warning-type way to say they need to be careful there.

Is måste here a strong obligation like English “must”, or can it also mean “should”?

Måste usually expresses a fairly strong necessity or obligation, similar to English must / have to:

Jag måste gå nu. – I have to go now.
Du måste göra det här. – You have to do this.

In practice, just like in English, people sometimes soften it in context so it can feel closer to really should:

Barnen måste passa sig vid sjön.
→ They really need to be careful at the lake (there is real danger).

A clearly softer alternative would be bör (should), but that is more formal and less common in everyday speech:
Barnen bör passa sig vid sjön. – The children should be careful by the lake.

How would the sentence change with a different subject, like “you” or “we”?

You keep måste and change the subject and reflexive pronoun:

Jag måste passa mig vid sjön. – I must be careful by the lake.
Du måste passa dig vid sjön. – You must be careful by the lake.
Han/Hon måste passa sig vid sjön. – He/She must be careful by the lake.
Vi måste passa oss vid sjön. – We must be careful by the lake.
Ni måste passa er vid sjön. – You (plural) must be careful by the lake.
De måste passa sig vid sjön. – They must be careful by the lake.

Notice that only the reflexive pronoun changes; måste and passa stay the same.