Vi packar en matsäck med smörgåsar och äpplen när vi ska till stranden.

Because matsäck is a common-gender noun in Swedish. Common-gender nouns take en in the singular indefinite form.

So:

• en matsäck = a packed meal / a picnic
• matsäcken = the packed meal / the picnic

Swedish often uses the present tense where English also uses the simple present for habits or regular actions.

So Vi packar ... när vi ska till stranden can mean something like We pack ... when we go to the beach or whenever we’re going to the beach.

Swedish also often uses the present tense for future situations when the context is clear, so this form is very normal.

Here ska means something like are going to, intend to, or are about to.

So vi ska till stranden means we’re going to the beach.

This is very natural Swedish. A motion verb such as gå or åka does not need to be stated, because it is understood from the context.

Because till shows movement toward a destination, while på shows location.

• till stranden = to the beach
• på stranden = at the beach / on the beach

So:

• vi ska till stranden = we are going to the beach
• vi är på stranden = we are at the beach

Because stranden is the definite form, meaning the beach.

In Swedish, the definite article is often attached to the end of the noun:

• en strand = a beach
• stranden = the beach

Here the definite form sounds natural because the beach is treated as a specific, known destination.

In Swedish, indefinite plural nouns usually have no article.

So:

• smörgåsar = sandwiches
• äpplen = apples

English sometimes uses some, but Swedish often just uses the plural noun by itself:

• med smörgåsar och äpplen = with sandwiches and apples

It often is, but not always. In äpplen, the -en is part of the plural form of äpple.

The forms are:

• ett äpple = an apple
• äpplet = the apple
• äpplen = apples
• äpplena = the apples

So äpplen is indefinite plural, not definite singular.

Yes. In modern Swedish, smörgås is a normal word for sandwich.

It can sometimes suggest an open sandwich in traditional contexts, but in everyday use it often just means sandwich. Another very common everyday word is macka, but smörgås is completely natural here.

Here med means with, but more specifically it shows what the matsäck contains.

So:

• en matsäck med smörgåsar och äpplen = a packed meal with sandwiches and apples

It is describing the contents of the food you pack.

After när, Swedish uses subordinate clause word order, so the subject comes before the verb:

• när vi ska till stranden

That is why it is vi ska, not something else.

If you move the när clause to the beginning of the sentence, the main clause changes word order because Swedish follows the V2 rule in main clauses:

• När vi ska till stranden packar vi en matsäck.

Notice that it becomes packar vi, not vi packar.

Yes. Those versions are more specific about how you get there:

• går till stranden = walk to the beach
• åker till stranden = go to the beach by car, bus, bike, etc.
• ska till stranden = are going to the beach, without saying how

So ska till stranden is a general and very natural way to say it.