Det var inte förrän jag öppnade min inkorg som jag såg att hennes mejl låg i skräppost.

This is a very common Swedish pattern used for emphasis. It means:

• It was not until ... that ...

So:

• Det var inte förrän jag öppnade min inkorg som jag såg ...
• It wasn’t until I opened my inbox that I saw ...

How it is built:

• Det var = It was
• inte förrän = not until
• som = that in this kind of emphatic structure

A simpler, less emphatic version would be:

• Jag såg inte att hennes mejl låg i skräppost förrän jag öppnade min inkorg.
• I didn’t see that her email was in spam until I opened my inbox.

The original version puts extra focus on the moment of opening the inbox.

Here, det is mostly a grammatical placeholder, like it in English It was not until...

It does not refer to a specific thing. It is just part of the fixed structure:

• Det var inte förrän ... som ...

This is similar to English cleft sentences such as:

• It was John who called.
• It was yesterday that I found it.
• It wasn’t until later that I understood.

So in this sentence, det is not something you should translate literally as a concrete it.

In this structure, som links the emphasized part to the rest of the sentence.

Compare:

• Det var igår som jag ringde.
  • It was yesterday that I called.
• Det var inte förrän jag öppnade min inkorg som jag såg ...
  • It wasn’t until I opened my inbox that I saw ...

So som here works a lot like English that in this kind of sentence.

It is not the same as the som used in relative clauses like:

• mannen som bor där = the man who lives there

Even though it is the same word, its role here is tied to the det var ... som ... emphasis pattern.

Because the whole sentence describes a past situation.

• var = was
• öppnade = opened
• såg = saw
• låg = was lying / was sitting / was located

The timeline is:

1. I opened my inbox.
2. Then I noticed her email was in the spam folder.

Swedish uses the simple past here just as English often does in storytelling.

A present-tense version would be:

• Det är inte förrän jag öppnar min inkorg som jag ser att hennes mejl ligger i skräppost.

That would mean something like:

• It’s not until I open my inbox that I see that her email is in spam.

Because förrän is the normal word used after a negative in the sense of until / before.

• inte förrän = not until

Examples:

• Jag kom inte hem förrän klockan tio.
  • I didn’t get home until ten.
• Hon förstod det inte förrän senare.
  • She didn’t understand it until later.

By contrast, innan usually means before in a more straightforward way:

• Jag åt innan jag gick.
  • I ate before I left.

So in your sentence, inte förrän is exactly the right expression for not until.

Usually, förrän appears in negative contexts.

The most common pattern is:

• inte förrän = not until

But it can also appear with other negative words:

• ingen ... förrän
• aldrig ... förrän

Examples:

• Ingen gick hem förrän mötet var slut.
  • No one went home until the meeting was over.

So the important idea is that förrän normally goes with some kind of negation or negative meaning.

Because som jag såg is a subordinate clause, and Swedish subordinate clauses do not use main-clause V2 word order.

In a main clause, Swedish usually puts the finite verb in second position:

• Jag såg det.
• Igår såg jag det.

But after subordinating words like som and att, the normal order is:

• subject + verb

So:

• som jag såg
• att hennes mejl låg i skräppost

not:

• som såg jag
• att låg hennes mejl ...

This is a key Swedish grammar point:

• Main clause: V2 word order
• Subordinate clause: subject before the finite verb

Because that whole part is a content clause: it tells you what I saw.

• jag såg att hennes mejl låg i skräppost
• I saw that her email was in spam

Here, att means that.

It introduces the statement that was noticed:

• att hennes mejl låg i skräppost

In English, that can often be omitted:

• I saw (that) her email was in spam

In Swedish, att is often kept, especially in clear written language.

Swedish often uses ligga for things that are located somewhere, especially objects, items, or things sitting in a place or folder.

So:

• mejlet låg i skräppost
• literally: the email lay in spam
• natural English: the email was in spam / had ended up in spam

This use of ligga is very common in Swedish:

• Nycklarna ligger på bordet.
  • The keys are on the table.
• Filen ligger i en mapp.
  • The file is in a folder.

Using vara is not always wrong in every context, but ligga sounds more natural when talking about where a thing is located.

If you wanted to stress how it got there, you might say:

• hennes mejl hade hamnat i skräppost
  • her email had ended up in spam

That focuses more on the result of a previous event.

Because the sentence is describing the situation as it was at the moment of discovery.

• jag såg att hennes mejl låg i skräppost
• I saw that her email was in spam

This presents the email’s location as a state that was true at that time.

If you say:

• jag såg att hennes mejl hade hamnat i skräppost

that means:

• I saw that her email had ended up in spam

This version emphasizes that the email had previously been moved there.

So:

• låg = describes where it was
• hade hamnat = emphasizes how it got there / that it had already ended up there

Both can work, but they are slightly different in focus.

Inkorg is the standard Swedish word for inbox.

• in- = in
• korg = basket
• so literally it is something like in-basket, just like English inbox

This is the normal word for an email inbox.

Also:

• mejl is the common Swedish spelling of mail/email
• e-post is also common, but it is a bit more formal or neutral in some contexts

So:

• hennes mejl = her email / her message
• min inkorg = my inbox
• skräppost = spam / junk mail

All three are very natural choices in everyday Swedish.

In Swedish, names of categories, folders, and certain abstract locations often appear without the definite article after a preposition.

So:

• i skräppost = in spam / in the spam folder

This is similar to how Swedish often says things like:

• på sjukhus = in hospital / at the hospital
• i skolan or på jobbet depending on the expression

With email systems, skräppost can behave almost like the name of a folder or category, so the article is often omitted.

You may also hear:

• i skräppostmappen
  • in the spam folder

That version is more explicit.

Usually, no. In this sentence, när is not needed.

The standard pattern is:

• Det var inte förrän jag öppnade min inkorg som ...

Adding när would sound awkward or nonstandard to many speakers.

Think of förrän here as already introducing the time relationship. So Swedish normally says:

• inte förrän jag öppnade ...

not:

• inte förrän när jag öppnade ...

Yes. A more straightforward version is:

• Jag såg inte att hennes mejl låg i skräppost förrän jag öppnade min inkorg.

This means the same thing, but it is less emphatic.

Compare the feel:

• Det var inte förrän ... som ...
  • more focused, more dramatic, more literary or deliberate
• Jag såg inte ... förrän ...
  • simpler, more direct, very common in speech and writing

Both are correct.

Here it is singular.

• hennes mejl = her email / her message

The word mejl can be both singular and plural in form in many contexts, so the form itself does not always tell you the number.

But in this sentence, the verb låg is singular in meaning because it refers to one email being in spam.

If you wanted to make the plural clearer, context would usually do it:

• hennes mejl låg i skräppost
  • could theoretically mean her emails were in spam in another context

But here, most learners would naturally understand it as her email.