Det var inte förrän vi öppnade matsäcken som vi märkte att hon hade glömt juicen hemma.

This is a very common Swedish structure meaning It wasn’t until ... that ....

So:

Det var inte förrän vi öppnade matsäcken som vi märkte att hon hade glömt juicen hemma.

means literally:

It was not until we opened the picnic basket/lunch pack that we noticed that she had forgotten the juice at home.

The pattern is:

• Det var inte förrän X som Y
• It wasn’t until X that Y

Examples:

• Det var inte förrän igår som jag förstod. = It wasn’t until yesterday that I understood.
• Det var inte förrän hon ringde som vi blev oroliga. = It wasn’t until she called that we got worried.

In this pattern, som introduces the main event that happened after the delay.

So in:

• Det var inte förrän vi öppnade matsäcken som vi märkte ...

the part after som is the thing that finally happened: we noticed ...

You can think of som here as a required part of the fixed expression inte förrän ... som ...

It is not being translated word-for-word in natural English, but it is part of how Swedish builds this sentence.

Öppnade is the past tense of öppna = to open.

• vi öppnar = we open / are opening
• vi öppnade = we opened
• vi har öppnat = we have opened
• vi hade öppnat = we had opened

In the sentence, the action is a completed event in the past:

• vi öppnade matsäcken = we opened the lunch pack

So simple past is the normal choice.

Matsäck means food you take with you, especially for an outing, school trip, hike, or picnic. Depending on context, English translations could be:

• packed lunch
• lunch pack
• picnic food
• sometimes picnic basket, though that is not always literal

Here, matsäcken is in the definite form, so it means:

• the packed lunch
• the lunch bag/food they had with them

It does not necessarily mean a basket in the literal sense. It is more about the food brought along.

The -en ending is the definite article attached to the noun.

• en matsäck = a packed lunch
• matsäcken = the packed lunch

Swedish usually adds the definite article to the end of the noun instead of using a separate word like the.

Other examples:

• en bok = a book
• boken = the book
• en juice = a juice
• juicen = the juice

Here märkte means noticed or realized.

• märka can mean notice
• in some contexts it can also be close to realize or become aware

So:

• vi märkte att hon hade glömt juicen hemma = we noticed / realized that she had forgotten the juice at home

It suggests that the discovery happened at that moment, when they opened the lunch pack.

Hade glömt is the past perfect (pluperfect), meaning had forgotten.

That is used because her forgetting happened before the moment when we noticed it.

Timeline:

1. She forgot the juice at home.
2. Later, we opened the lunch pack.
3. Then we noticed it.

So Swedish uses:

• hon hade glömt = she had forgotten

If you said hon glömde, it would sound more like a simple past event, without clearly showing that it happened earlier than the noticing.

It is formed with:

• hade = had
• the supine form of the verb = glömt

So:

• glömma = to forget
• glömde = forgot
• glömt = forgotten
• hade glömt = had forgotten

This is the standard way to form the past perfect in Swedish.

Compare:

• hon har glömt = she has forgotten
• hon hade glömt = she had forgotten

Juicen means the juice.

Swedish often uses the definite form when talking about a specific thing that both speaker and listener understand from context.

Here, it is the specific juice that was supposed to be in the lunch pack.

• juice = juice
• juicen = the juice

A native English speaker might expect no article in some situations, but Swedish often prefers the definite form when the item is identifiable.

Hemma means at home, while hem usually means home as a direction.

So:

• hon är hemma = she is at home
• hon gick hem = she went home
• hon hade glömt juicen hemma = she had forgotten the juice at home

In this sentence, the juice was left at home as a location, so hemma is correct.

Because after att, Swedish uses subordinate clause word order.

So the normal order is:

• att + subject + verb
• att hon hade glömt

not:

• att hade hon glömt

This is very important in Swedish. In subordinate clauses, the subject usually comes before the finite verb.

Examples:

• Jag vet att hon kommer. = I know that she is coming.
• Vi märkte att hon hade glömt juicen hemma. = We noticed that she had forgotten the juice at home.

Yes. A more straightforward version would be:

• Vi märkte inte att hon hade glömt juicen hemma förrän vi öppnade matsäcken.

That means the same thing: We didn’t notice that she had forgotten the juice at home until we opened the lunch pack.

The original version with Det var inte förrän ... som ... is a bit more structured and slightly more emphatic, but both are natural.

It is very common and natural in both spoken and written Swedish.

It is not overly formal. Native speakers use it regularly when they want to emphasize that something happened only after a certain point.

Examples:

• Det var inte förrän då som jag förstod.
• Det var inte förrän senare som hon berättade sanningen.

So this is definitely a useful pattern to learn.