Jag vet inte om den regeln gäller på söndagar.

Jag vet inte om... means I don’t know if / whether...

Here:

• Jag = I
• vet = know
• inte = not
• om = if / whether

So Jag vet inte om den regeln gäller på söndagar means I don’t know if/whether that rule applies on Sundays.

In Swedish, om is very commonly used to introduce an indirect yes/no clause:

• Jag undrar om det är sant. = I wonder if it is true.
• Hon frågade om jag kom. = She asked whether I was coming.

Here, om means if / whether, not about.

Swedish om can mean different things depending on context:

• if / whether: Jag vet inte om...
• about: en bok om Sverige = a book about Sweden

So in this sentence, om introduces a clause about uncertainty: I don’t know whether...

Den regeln usually means that rule.

Compare:

• regeln = the rule
• den regeln = that rule

Swedish often uses den/det/de to point something out, like English that/those.

So:

• Jag vet inte om regeln gäller på söndagar. = I don’t know if the rule applies on Sundays.
• Jag vet inte om den regeln gäller på söndagar. = I don’t know if that rule applies on Sundays.

This makes it sound like the speaker is referring to a particular rule already mentioned or understood from context.

That is normal in Swedish when using a demonstrative like den with a noun.

• en regel = a rule
• regeln = the rule
• den regeln = that rule

So unlike English, Swedish often uses both:

1. a demonstrative word (den)
2. the definite form on the noun (regeln)

This is a standard pattern:

• den bilen = that car
• den dagen = that day
• den regeln = that rule

Gäller comes from gälla, which often means:

• to apply
• to be valid
• to be in force

So den regeln gäller means:

• that rule applies
• that rule is valid
• that rule is in effect

Examples:

• Regeln gäller inte längre. = The rule no longer applies.
• Detta gäller alla. = This applies to everyone.

Yes. Gäller is the present tense of gälla.

Swedish verbs are nice in one important way: they do not change depending on the subject.

So:

• jag gäller would not make sense semantically here, but the verb form itself stays the same in present tense
• regeln gäller
• de gäller if the meaning worked

A few forms of gälla:

• infinitive: gälla
• present: gäller
• past: gällde
• supine: gällt

In this sentence, present tense is used because the speaker is talking about whether the rule generally applies.

Swedish uses på with days of the week.

• på söndag = on Sunday / this coming Sunday
• på söndagen = on the Sunday / during Sunday, in a more specific sense
• på söndagar = on Sundays, habitually / generally

So på söndagar is the natural choice here because the sentence is about what happens regularly on Sundays in general.

Examples:

• Butiken är stängd på söndagar. = The shop is closed on Sundays.
• Vi ses på söndag. = We’ll see each other on Sunday.

Because the sentence means on Sundays in a general, repeated sense.

In Swedish, just like in English, plural is often used for recurring time expressions:

• på måndagar = on Mondays
• på helgerna = on the weekends
• på söndagar = on Sundays

If you said på söndag, that would usually mean this coming Sunday or on Sunday as one specific occasion.

Because after om you have a subordinate clause, and subordinate clauses normally use subject + verb word order.

So:

• main clause: Jag vet inte...
• subordinate clause: om den regeln gäller på söndagar

That gives:

• den regeln = subject
• gäller = verb

Compare:

Main clause question

• Gäller den regeln på söndagar? = Does that rule apply on Sundays?

Subordinate clause

• Jag vet inte om den regeln gäller på söndagar. = I don’t know if that rule applies on Sundays.

So inversion (gäller den regeln) is used in direct questions, but not here.

Because in a main clause, Swedish usually places inte after the finite verb.

Here the main clause is:

• Jag vet inte = I do not know

So the pattern is:

• subject + verb + inte

Examples:

• Jag kommer inte. = I’m not coming.
• Hon förstår inte. = She doesn’t understand.

If the negation were inside the subordinate clause, its position would be different:

• Jag vet att den regeln inte gäller på söndagar. = I know that that rule does not apply on Sundays.

Notice:

• main clause: vet inte
• subordinate clause: regeln inte gäller

That difference is very important in Swedish word order.

Yes, often you can.

• Jag vet inte om den regeln gäller på söndagar.
• Jag vet inte ifall den regeln gäller på söndagar.

Both can mean I don’t know if/whether that rule applies on Sundays.

However, om is more basic and very common, so it is the safest choice for learners.

The dictionary forms are:

• regel = rule
• söndag = Sunday

In the sentence:

• regeln = regel in definite singular
• söndagar = söndag in indefinite plural

So the forms are:

• en regel = a rule
• regeln = the rule
• söndag = Sunday
• söndagar = Sundays

It is talking about Sundays in general.

That is because of på söndagar, which signals a habitual or repeated meaning:

• Jag vet inte om den regeln gäller på söndagar. = I don’t know if that rule applies on Sundays generally.

If you wanted one specific Sunday, you would more likely say:

• Jag vet inte om den regeln gäller på söndag. = I don’t know if that rule applies on Sunday.

So the plural form is what makes the sentence sound general and recurring.