Tjuven stal smyckena.

Because tjuven is the definite singular form of tjuv, meaning the thief.

Swedish often puts the definite article on the end of the noun instead of using a separate word like English the.

• en tjuv = a thief
• tjuven = the thief

Since tjuv is an en-word (common gender noun), its definite singular ending is -en.

Because smyckena is the definite plural form of smycke.

Its forms are:

• ett smycke = a piece of jewelry / an ornament
• smycket = the piece of jewelry
• smycken = pieces of jewelry / jewels
• smyckena = the pieces of jewelry / the jewels

The ending -na here marks definite plural: the jewelry / the jewels.

In Swedish, the definite article is usually attached to the noun as a suffix.

So instead of saying something like the thief with a separate word, Swedish says:

• tjuven = the thief
• smyckena = the jewels / the jewelry

This is one of the biggest differences from English.

A useful extra point: if there is an adjective, Swedish often uses both a separate article and the ending:

• den gamla tjuven = the old thief

So in simple cases, the article is usually attached to the noun, but with adjectives the pattern becomes a little fuller.

Stal is past tense.

The verb is att stjäla, meaning to steal. Its main forms are:

• att stjäla = infinitive
• stjäl = present
• stal = past
• stulit = supine / past participle-related form used in perfect constructions

So:

• Tjuven stjäl smyckena = The thief steals the jewelry
• Tjuven stal smyckena = The thief stole the jewelry
• Tjuven har stulit smyckena = The thief has stolen the jewelry

It is an irregular verb, so you cannot predict stal just by adding a regular ending.

The dictionary forms are:

• tjuv = thief
• stjäla = to steal
• smycke = piece of jewelry / ornament

The sentence uses inflected forms:

• tjuven = definite singular of tjuv
• stal = past tense of stjäla
• smyckena = definite plural of smycke

When learning Swedish, it is very useful to mentally strip words back to their base forms like this.

That is a very common question.

English often uses jewelry as an uncountable mass noun. Swedish more naturally uses the plural noun smycken or smyckena when talking about pieces of jewelry.

So:

• smycken = jewels / pieces of jewelry
• smyckena = the jewels / the pieces of jewelry

Depending on context, English may translate smyckena as either:

• the jewels
• the jewelry

So the Swedish grammar is plural, even if the most natural English translation may look singular.

The basic word order here is:

subject + verb + object

• Tjuven = subject
• stal = verb
• smyckena = object

So it works very much like standard English word order in a simple statement:

• The thief
  • stole
    • the jewels

However, Swedish has an important rule called V2: in main clauses, the finite verb usually comes in the second position. So if you move another element to the front, the verb still stays second:

• Igår stal tjuven smyckena. = Yesterday, the thief stole the jewelry.

Notice that stal is still in second position.

You would use the indefinite forms:

• En tjuv stal smycken.

That means A thief stole jewelry / jewels.

Compare:

• Tjuven stal smyckena. = The thief stole the jewelry
• En tjuv stal smycken. = A thief stole jewelry

A small nuance: smycken without the definite ending sounds more like jewelry/jewels in general, while smyckena refers to specific jewelry already known from the context.

It can mean either, depending on how natural the English translation is in context.

Grammatically, smyckena is plural definite, so a very direct translation is the jewels or the pieces of jewelry.

But in natural English, people often say the jewelry instead, especially when speaking about stolen valuables as a group.

So both can be right:

• more literal: the jewels
• often more natural in context: the jewelry

The hardest parts for English speakers are usually tj in tjuven and y in smyckena.

A few helpful points:

• tj in tjuven is a soft Swedish sound with no exact English equivalent. It is somewhat similar to sh, but not exactly the same.
• y in smyckena is a rounded front vowel, like German ü or French u. English does not really have this sound.
• Stress falls on the first syllable in both tjuven and smyckena.

Also:

• tjuven is pronounced with a soft beginning, not like an English t
• stal has a long vowel, roughly like stahl
• ck in smyckena signals a short vowel before the k sound

If you are learning pronunciation, it is best to hear this from native audio, because these sounds are difficult to learn from spelling alone.

To make it negative, add inte:

• Tjuven stal inte smyckena. = The thief did not steal the jewelry.

To make it a yes/no question, Swedish usually puts the verb first:

• Stal tjuven smyckena? = Did the thief steal the jewelry?

To ask who did it:

• Vem stal smyckena? = Who stole the jewelry?

These are useful transformations because they show that stal is the finite verb and that Swedish changes word order rather than using do like English does.

Because Swedish nouns belong to different grammatical genders:

• en tjuv: common gender
• ett smycke: neuter gender

This affects articles and endings.

For tjuv:

• en tjuv
• tjuven

For smycke:

• ett smycke
• smycket
• smycken
• smyckena

Unlike English, you have to learn the noun together with en or ett, because that choice affects the whole pattern of the word.