På söndagar säljer de jordgubbar på marknaden.

På söndagar means on Sundays / every Sunday, so it describes a repeated habit.

• på söndag = on Sunday (one specific upcoming Sunday)
• på söndagar = on Sundays (regularly, in general)

So this sentence is about something that happens repeatedly.

Because Swedish often uses the plural to talk about something that happens regularly on a certain day.

So:

• på söndagar = on Sundays
• på måndagar = on Mondays
• på kvällarna can mean in the evenings

It works a lot like English here, where we also often use the plural for routines: On Sundays, they sell strawberries...

This is because of the Swedish V2 rule: in a main clause, the finite verb usually comes in second position.

Here, På söndagar is placed first. That means the verb must come next:

• På söndagar
  • säljer
    • de
      • ...

If you started with the subject instead, you would get:

• De säljer jordgubbar på marknaden på söndagar.

Both are grammatical, but the original sentence puts emphasis on when it happens.

De means they.

In this kind of sentence, it can refer to:

• a specific group already known from context, or
• people in general, such as the vendors or sellers

So it may be a bit like English they when the exact identity is not important.

Also, in everyday spoken Swedish, de is usually pronounced like dom.

Säljer is the present tense of sälja (to sell).

Common forms:

• att sälja = to sell
• säljer = sell / are selling
• sålde = sold
• sålt = sold (supine/past participle-related form in compound tenses)

So de säljer = they sell / they are selling, depending on context.

Because jordgubbar is an indefinite plural noun here: strawberries.

Swedish often leaves out an article in the plural when English does too:

• De säljer jordgubbar. = They sell strawberries.

Compare:

• en jordgubbe = a strawberry
• jordgubben = the strawberry
• jordgubbar = strawberries
• jordgubbarna = the strawberries

So this sentence means they sell strawberries in general, not a specific known batch of strawberries.

In Swedish, på is often used for places and events where English might say at.

So:

• på marknaden = at the market

Using i would suggest being physically inside something, and with marknad that is usually not the natural choice.

This is a common Swedish pattern:

• på jobbet = at work
• på banken = at the bank
• på en konsert = at a concert
• på marknaden = at the market

Because the same preposition can be used in different expressions.

Here the two på have different functions:

• på söndagar = time expression (on Sundays)
• på marknaden = place expression (at the market)

This is normal in Swedish, just as English can repeat on or at in one sentence:

• On Sundays, they are at the market.

Marknaden means the market.

The definite article in Swedish is usually added as an ending:

• en marknad = a market
• marknaden = the market

In this sentence, it likely refers to a known or specific market, perhaps the local market that both speaker and listener know about.

Yes, absolutely.

That version is also correct:

• De säljer jordgubbar på marknaden på söndagar.

The difference is mainly focus:

• På söndagar säljer de... emphasizes when
• De säljer... på söndagar starts more neutrally with they

Both mean essentially the same thing.

In standard written Swedish, de is the normal spelling for they.

But in speech, many people pronounce both de and dem as dom.

So:

• written: de säljer
• spoken: often sounds like dom säljer

You should still learn the standard written forms:

• de = they
• dem = them

But it is useful to know that spoken Swedish often sounds different.

Literally, jordgubbe is made up of:

• jord = earth/soil
• gubbe = old man / fellow / little man (depending on context)

But as a whole word, jordgubbe simply means strawberry. You should learn it as one vocabulary item rather than trying to understand it from the parts.