Learn Hardcore Swedish: På vintern lägger jag en tjock filt över barnens lakan. - In the winter I put an extra thick blanket over the children's sheets.

På vintern lägger jag en tjock filt över barnens lakan.
Starts with the time expression. This is very common in Swedish; time/place adverbials often come first. The structure is:
Adverbial – Verb – Subject – (Object …)
→ På vintern | lägger | jag | en tjock filt …
Jag lägger en tjock filt över barnens lakan på vintern.
Here the subject jag comes first. This is also fine and quite neutral.

In both cases, the finite verb must be in second position in main clauses (the V2 rule), so if you move på vintern to the front, the subject has to move after the verb: På vintern lägger jag …, not På vintern jag lägger ….

Why is it på vintern and not i vintern?

For seasons and many recurring time periods, Swedish usually uses på:

på vintern – in (the) winter
på sommaren – in (the) summer
på kvällen – in the evening
på måndagar – on Mondays

Using i vintern is ungrammatical in this general-time sense.

You could also say på vintern without the definite article in English (“in winter”), but in Swedish the definite vintern is standard here.

What exactly does lägger mean here? Why not sätter or ställer?

Swedish uses different verbs depending on how you place something:

lägga – to lay/put something in a lying, horizontal position
ställa – to put something so it stands, vertical/standing
sätta – to set/put something so it sits/is attached (often people or objects that “sit”)

A blanket is spread out in a lying position on a bed, so you use lägga:

lägga en filt på sängen – lay/put a blanket on the bed

Using sätter or ställer with a blanket would sound wrong or at least very odd.

Why is it en tjock filt and not den tjocka filten?

en tjock filt = a thick blanket (indefinite, introducing it as any blanket)
den tjocka filten = the thick blanket (definite, a specific blanket you both know about)

In the sentence, the speaker is talking about what they usually do in winter in general, not about some particular already-known blanket, so the indefinite form is natural.

You could use den tjocka filten if, for example, there is one special winter blanket that everyone in the context already knows:

På vintern lägger jag den tjocka filten över barnens lakan.
→ In the winter I put the thick blanket (that we always use) over the children’s sheets.

Why does the adjective change like tjock – tjockt – tjocka? What form is tjock in en tjock filt?

Swedish adjectives agree with the noun in gender (en/ett) and number (singular/plural), and also with definiteness.

For tjock:

en-words, singular indefinite: en tjock filt
ett-words, singular indefinite: ett tjockt lakan
plural (both genders), indefinite: tjocka filtar, tjocka lakan
definite (singular or plural): den tjocka filten, det tjocka lakanet, de tjocka filtarna

In the sentence, filt is an en-word in the indefinite singular, so the adjective stays in its base form: en tjock filt.

Why do we use över and not på before barnens lakan?

Both över and på can be used with beds and sheets, but they have slightly different images:

över = over, on top of, covering
→ en filt över lakanen suggests the blanket covers the sheets.
på = on (in contact with, on the surface)
→ en filt på lakanen is more like “a blanket on the sheets” without explicitly emphasising the idea of covering.

Here, the idea is that the thick blanket covers the children’s sheets, so över is very natural.

What is happening grammatically in barnens lakan? Why is there an -s on barnen?

Barnens is the genitive (possessive) form of barn (child/children):

ett barn – a child
barn – child / children (indefinite; the form is the same)
barnen – the child / the children (definite)
barnens – the child’s / the children’s (possessive)

In Swedish, you add -s directly to the noun to show possession:

barnens lakan – the children’s sheets
mammas bok – mum’s book
Johans bil – Johan’s car

There is no apostrophe in Swedish. You do not write barnen’s.

So barnens lakan literally means “the children’s sheets”.

Why doesn’t lakan change form here? Is this singular or plural?

Lakan is an ett-word with the same form in indefinite singular and indefinite plural:

singular indefinite: ett lakan – a sheet
plural indefinite: lakan – sheets
singular definite: lakanet – the sheet
plural definite: lakanen – the sheets

In barnens lakan, the context (children, each with sheets) makes a plural reading natural: the children’s sheets.

Formally, lakan on its own doesn’t show whether it’s singular or plural; you see that from the article (ett / –) or from context.

Why is the verb in the present tense (lägger) if this is about what happens every winter?

Swedish uses the present tense not only for right now, but also for:

habitual actions (things you usually do)
general truths
some future references

So:

På vintern lägger jag en tjock filt över barnens lakan.
→ In the winter I put / I usually put a thick blanket over the children’s sheets.

Even in English, we often use the simple present for habits (“I always wear a coat in winter”). Swedish does the same, so there’s no special “habitual tense”; present covers this use.

Could I say över deras lakan instead of över barnens lakan? What’s the difference?

Yes, both are grammatically correct, but there’s a nuance:

över barnens lakan – the children’s sheets, using a noun + -s genitive.
This is the most natural way if “the children” are already known in the context.
över deras lakan – over their sheets, using a pronoun (deras = their).
This sounds a bit more distant or less specific; it doesn’t repeat the noun “children”.

If you have already just mentioned barnen in the previous sentence, both could work:

Barnen fryser lätt. På vintern lägger jag en tjock filt över deras lakan.
Barnen fryser lätt. På vintern lägger jag en tjock filt över barnens lakan.

The version with barnens more clearly links the sheets to these specific children.

How do you pronounce tricky parts like tjock, barnens, and the vowels å, ä?

Very briefly:

å in på: like English “or” but shorter and more rounded.
ä in vintern, barnens: similar to the vowel in English “bed”, but a bit clearer and tenser.

Specific words:

tjock: tj- here is usually pronounced like a soft “sh” sound (IPA [ɕ]), something between “sh” and “ch”: [ɕɔk].
barnens:
- b as in English
- ar is long, a bit like “baarn”
- nens is straightforward: “nens”.
  So roughly “BAAR-nens”, with a long a.

Swedish stress is on the first syllable in vintern, lägger, barnens, lakan.