Kuvertet gick sönder när jag öppnade det.

Because kuvertet means the envelope, while kuvert means an envelope or just envelope in a dictionary form.

Kuvert is a neuter noun in Swedish, so its definite singular form takes -et:

• ett kuvert = an envelope
• kuvertet = the envelope

So the sentence is talking about a specific envelope, not just any envelope.

Det means it, and it refers back to kuvertet.

So:

• Kuvertet gick sönder = The envelope broke / tore
• när jag öppnade det = when I opened it

In English, we also usually repeat the object with it: when I opened it, not just when I opened.

Because kuvert is a neuter noun (ett kuvert), the pronoun is det.

Gick sönder is a very common Swedish expression meaning broke, came apart, got damaged, or in this context tore.

Literally, gick means went, and sönder means apart / broken, so the literal idea is something like went apart. But you should learn gå sönder as a set expression.

It is used for many things:

• Glaset gick sönder. = The glass broke.
• Mobilen gick sönder. = The phone broke.
• Kuvertet gick sönder. = The envelope tore / came apart.

Yes, gick is the past tense of gå, but gå is also used in several idiomatic expressions where it does not mean walk.

One of those expressions is gå sönder, which means break or come apart.

So in this sentence:

• gick is not about movement
• it is simply part of the fixed expression gå sönder

This is similar to how English uses verbs in idiomatic ways that are not fully literal.

Swedish verbs do not change according to the subject the way English verbs sometimes do.

So:

• jag öppnade = I opened
• du öppnade = you opened
• han/hon öppnade = he/she opened
• vi öppnade = we opened

The verb form öppnade stays the same for all persons in the past tense.

That is much simpler than in many other languages.

Because när jag öppnade det is a subordinate clause, and Swedish subordinate clauses do not use the main-clause V2 word order.

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

• Jag öppnade det. = I opened it.

But after a subordinating word like när (when), the normal order is:

• när jag öppnade det

So:

• Jag öppnade det. = main clause
• när jag öppnade det = subordinate clause

Using när öppnade jag det would sound like a direct question: when did I open it?

Here när means when.

The sentence means that the envelope tore at the time you were opening it. In natural English, that can sometimes overlap with while, but the Swedish word here is simply när.

Compare:

• när jag öppnade det = when I opened it

If you wanted to strongly emphasize an ongoing action like while I was opening it, Swedish might sometimes use other wording depending on context, but när is completely normal here.

That version is also possible, but it gives a slightly different structure and emphasis.

• Kuvertet gick sönder när jag öppnade det.
  Focuses first on what happened to the envelope.

• Jag öppnade kuvertet och det gick sönder.
  Focuses first on your action.

The original sentence is very natural because the main point is that the envelope tore.

It is very natural here.

Even though envelopes do not usually break in the same way a glass or phone does, Swedish still commonly uses gå sönder for something becoming damaged or coming apart unexpectedly.

For an envelope, English might prefer tore or ripped, but Swedish can still say:

• Kuvertet gick sönder

If you wanted to be more specific, you might use a verb related to tearing, but gick sönder is idiomatic and normal.

Because Swedish usually expresses definiteness with an ending on the noun, not with a separate word like the.

So instead of:

• the envelope

Swedish says:

• kuvertet

The -et ending already contains the idea of the.

The whole sentence is in the simple past.

• gick = went / broke
• öppnade = opened

So both actions are presented as completed past events:

• the envelope tore
• you opened it

This is very normal in Swedish storytelling and everyday speech.

Yes, that is also correct.

It starts with the subordinate clause:

• När jag öppnade det = When I opened it

Then the main clause follows:

• gick kuvertet sönder = the envelope tore / broke

Notice the inversion in the main clause after the fronted subordinate clause:

• När jag öppnade det gick kuvertet sönder.
• not När jag öppnade det kuvertet gick sönder.

This happens because Swedish main clauses follow the V2 rule: the finite verb comes in second position.

In this sentence, sönder is not the main verb. The verb is gick.

Sönder is an adverb-like word meaning something like apart, broken, or into pieces depending on context. In gå sönder, it combines with the verb to create the meaning break / come apart.

So it is best to learn gå sönder as one expression rather than analyzing sönder separately every time.