Om blixtlåset går sönder igen, måste hon sy fast det.

Swedish very often builds compound nouns as a single word, so blixtlås means zipper.

Here, blixtlåset is the definite singular form, meaning the zipper.

• ett blixtlås = a zipper
• blixtlåset = the zipper

The ending -et shows that blixtlås is a neuter noun, an ett-word.

Here, om means if.

It introduces a condition:

• Om blixtlåset går sönder igen ... = If the zipper breaks again ...

Swedish om can also mean whether in other contexts, so learners often need to rely on context.

Gå sönder is a very common Swedish expression meaning break, come apart, or stop functioning.

So blixtlåset går sönder means the zipper breaks or the zipper comes apart.

Even though går literally means goes, you should learn gå sönder as a fixed expression. It is used for things that stop working or become damaged.

Examples:

• Min telefon gick sönder. = My phone broke.
• Leksaken går lätt sönder. = The toy breaks easily.

Går sönder focuses on the event of becoming broken.

• går sönder = breaks, becomes damaged
• är trasig = is broken

So in this sentence, the idea is that the zipper might break again in the future, which is why går sönder is the natural choice.

You may hear är sönder in speech, but är trasig is usually more standard if you want to describe the state.

This is because Swedish follows the verb-second rule in main clauses.

The sentence starts with a subordinate clause:

• Om blixtlåset går sönder igen

After that, the main clause begins, and the finite verb must come first in that clause:

• måste hon sy fast det

So the order becomes:

• subordinate clause first
• then finite verb
• then subject

Compare:

• Hon måste sy fast det. = She has to sew it on.
• Om blixtlåset går sönder igen, måste hon sy fast det. = If the zipper breaks again, she has to sew it on.

Måste means must or has to.

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

So:

• måste sy = must sew
• not måste att sy

This works like English:

• must sew
• not must to sew

Other common Swedish modal verbs work the same way:

• kan sy = can sew
• vill sy = wants to sew
• ska sy = will / is going to sew

Sy fast means sew on, sew down, or attach by sewing.

The word fast is a particle here, and together with sy it creates a particle verb.

So sy fast det means something like:

• sew it back on
• sew it in place
• attach it by sewing

This is very natural Swedish for fastening something with stitches.

Many Swedish verbs are particle verbs, where the main verb and the particle are written separately.

So:

• sy fast
• sätta på
• slå av
• komma in

In dictionaries, you will often find them listed together as a unit, but in normal writing they are usually separate words.

In this sentence:

• måste hon sy fast det

The infinitive is sy, and fast stays with it as the particle.

Det refers back to blixtlåset.

Since blixtlås is an ett-word, the matching pronoun is det.

So:

• ett blixtlås
• blixtlåset
• det

Even though English might sometimes avoid repeating the object, Swedish naturally keeps det here because it is the thing being sewn back on or attached.

Here, igen means again, and går sönder igen is the natural placement.

So:

• Om blixtlåset går sönder igen = If the zipper breaks again

This position sounds very normal because igen comes after the verbal idea it modifies.

You can sometimes move igen for emphasis in other sentences, but here the standard, neutral order is exactly what you see.

In this sentence, hon simply means she.

Swedish personal pronouns are straightforward here:

• jag = I
• du = you
• han = he
• hon = she
• det = it

So måste hon sy fast det means that she is the person who has to do the sewing.

Two things would usually change: the noun form and possibly the pronoun.

Here, blixtlås is a neuter noun, so the singular definite form is blixtlåset, and the pronoun is det.

If it were an en-word, you would normally use den instead of det.

For example:

• Om jackan går sönder igen, måste hon laga den.
  • jackan is an en-word
  • so the pronoun is den

If it were plural:

• Om blixtlåsen går sönder igen, måste hon sy fast dem.

So pronoun choice depends on the gender and number of the noun:

• den for common gender singular
• det for neuter singular
• dem for plural