Om det regnar i helgen, går vi till simhallen i stället för till stranden, där barnen annars vill simma.

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

Here, the whole clause Om det regnar i helgen is the first element. That means the finite verb of the main clause, går, must come next, before the subject vi:

• Om det regnar i helgen, går vi ...

If you started directly with the main clause, you would say:

• Vi går till simhallen ...

So the changed word order is caused by the opening if-clause.

In Swedish, weather expressions often use a dummy subject det, just like English uses it:

• det regnar = it is raining
• det snöar = it is snowing

The det does not refer to a specific thing. It is just required by the grammar.

I helgen usually means this weekend or over the weekend in a specific situation.

• Om det regnar i helgen ... = If it rains this weekend ...

På helgen usually means on weekends in a habitual or general sense.

• Vi tränar på helgen = We train on weekends.

So:

• i helgen = one particular weekend
• på helgen = weekends generally

Swedish very often uses the present tense for future meaning when the time is already clear from context.

Here, i helgen makes it clear that the action is in the future, so present tense sounds natural:

• Om det regnar i helgen, går vi ...

English can do something similar in some cases, but Swedish relies on this much more often.

I stället för is a fixed expression meaning instead of.

So:

• i stället för till stranden = instead of going to the beach

It is best learned as one chunk:

• i stället för = instead of

Because för belongs to the fixed expression i stället för. It is not the preposition that shows direction.

The verb gå uses till for a destination:

• gå till simhallen
• gå till stranden

So both destinations need till:

• går vi till simhallen i stället för till stranden

A good way to see it is:

• to the pool instead of to the beach

In Swedish, keeping the second till here is natural and clear.

Because they mean the swimming pool facility and the beach, not just any pool or any beach.

In Swedish, the definite form is often used when the place is understood from the situation:

• simhall = a swimming hall / indoor pool

• simhallen = the swimming hall / the pool facility

• strand = beach
• stranden = the beach

This can feel a little broader in Swedish than in English. Swedish often uses the definite form for familiar, contextually understood places.

Simhallen literally means the swimming hall.

In normal English, this is usually best understood as:

• the indoor swimming pool
• the public pool
• the swimming facility

It is a common Swedish word for a municipal or public place where people go swimming indoors.

Here där means where and refers back to stranden.

So this part:

• stranden, där barnen annars vill simma

means:

• the beach, where the children otherwise want to swim

It introduces extra information about the beach.

Here annars means something like otherwise, normally, or if not.

The idea is:

• if it were not for the rain / in other circumstances, the children would want to swim at the beach

So annars contrasts the actual plan with the usual or expected alternative.

Because this is a subordinate clause introduced by där.

In Swedish subordinate clauses, adverbs like annars often come before the finite verb:

• där barnen annars vill simma

Compare that with main clause word order, where the finite verb comes earlier because of the V2 rule.

So this difference is a very important pattern:

• main clause: verb early
• subordinate clause: subject + adverb + finite verb

Because vilja is a modal verb, and modal verbs in Swedish are followed by the infinitive without att.

So:

• vill simma = want to swim
• kan simma = can swim
• måste simma = must swim

This is similar to English, where we also say want to swim but can swim. Swedish modal patterns are different from English in detail, so it is useful to memorize that vilja + infinitive normally appears without att in this kind of sentence.

Not always. In modern Swedish, commas before subordinate clauses are often optional and depend partly on style and clarity.

So many writers might also write:

• Om det regnar i helgen går vi till simhallen i stället för till stranden där barnen annars vill simma.

The comma in your sentence helps show a pause and separates the extra descriptive part about the beach. It is natural, but not the only possible punctuation choice.