Värdesakerna låg i ett låst skåp.

Because -na is the definite plural ending.

• värdesak = a valuable
• värdesaker = valuables
• värdesakerna = the valuables

So Värdesakerna låg i ett låst skåp means the valuables were/lay in a locked cabinet.

In Swedish, the definite article is often attached to the end of the noun instead of being a separate word like the in English.

Literally, värdesak is made up of:

• värde = value
• sak = thing

So a värdesak is literally a valuable thing, which in natural English is usually a valuable or an item of value.

The plural värdesaker often corresponds to English valuables.

Låg is the past tense of ligga, which means to lie or to be lying.

• ligga → present: ligger → past: låg

In Swedish, verbs like ligga, stå, and sitta are often used where English might simply use be:

• Boken låg på bordet = The book was lying on the table
• Glasen stod i skåpet = The glasses were in the cabinet
• Han satt i bilen = He was sitting in the car

So here, låg tells you that the valuables were located there, with the idea of lying/stored inside the cabinet.

You could sometimes say var in Swedish, but låg sounds more natural here.

Compare:

• Värdesakerna var i ett låst skåp = The valuables were in a locked cabinet
• Värdesakerna låg i ett låst skåp = The valuables were lying/in storage in a locked cabinet

Using låg gives a more specific physical sense. Swedish often prefers this kind of verb when talking about where things are located.

So var is more neutral, while låg is more descriptive and idiomatic.

Because skåp is an ett-word in Swedish.

Swedish nouns have two grammatical genders/common article types:

• en words
• ett words

Since skåp takes ett, you say:

• ett skåp = a cabinet / a cupboard

That is why the sentence has i ett låst skåp.

Because the adjective/participle must agree with the noun it describes.

Here:

• skåp is an ett-word
• therefore the describing word takes -t

So:

• en låst dörr = a locked door
• ett låst skåp = a locked cabinet

Låst is the past participle of låsa = to lock.

So ett låst skåp literally means a locked cabinet.

It is a past participle, but in this sentence it functions like an adjective.

• låsa = to lock
• låst = locked

So in ett låst skåp, låst describes the cabinet in the same way that red describes a car in a red car.

This is very common in Swedish:

• en stängd dörr = a closed door
• ett öppet fönster = an open window
• ett låst skåp = a locked cabinet

Because Swedish usually puts the definite meaning on the noun itself.

So instead of a separate word like the, Swedish often uses an ending:

• värdesaker = valuables
• värdesakerna = the valuables

That means Swedish does not need a separate article before the noun here.

This is the normal Swedish main-clause word order:

subject + verb + other information

So:

• Värdesakerna = subject
• låg = verb
• i ett låst skåp = prepositional phrase telling where

This is very similar to standard English word order:

• The valuables
  • were/lay
    • in a locked cabinet

Because i means in/inside.

Since the valuables were inside the cabinet, Swedish uses i:

• i ett skåp = in a cabinet
• i en låda = in a drawer
• i väskan = in the bag

If the thing were on top of something, Swedish would more likely use på.

Skåp is a general word for a cabinet, cupboard, or sometimes a locker, depending on context.

So ett låst skåp could mean:

• a locked cabinet
• a locked cupboard
• a locked locker

The best English translation depends on the situation.

Yes. That would mean The valuables were in the locked cabinet.

Compare:

• i ett låst skåp = in a locked cabinet
• i det låsta skåpet = in the locked cabinet

So the original sentence does not point to one specific cabinet already known to the listener. It just says the valuables were in a locked cabinet.

The present tense would be:

Värdesakerna ligger i ett låst skåp.

• ligger = present tense
• låg = past tense

So:

• Värdesakerna ligger i ett låst skåp = The valuables are in a locked cabinet
• Värdesakerna låg i ett låst skåp = The valuables were in a locked cabinet

Not always. That is part of the idea, but Swedish uses ligga more broadly than English uses lie.

For objects, Swedish often chooses among:

• ligga = lie
• stå = stand
• sitta = sit

But the choice is often idiomatic, not purely logical from an English perspective.

For valuables stored in a cabinet, låg sounds natural even if you are not imagining each item literally lying flat. It simply describes their location in a typical Swedish way.