Tjejen väljer mobiloperatören.

Because both nouns are in the definite singular form.

In Swedish, the definite article is usually attached to the end of the noun instead of being a separate word.

• tjej = girl

• tjejen = the girl

• mobiloperatör = mobile operator
• mobiloperatören = the mobile operator

Both of these nouns are en-words (common gender), so the definite singular ending is usually -en.

Swedish normally puts the at the end of the noun as a suffix.

So instead of:

• the girl
• the mobile operator

Swedish says:

• tjejen
• mobiloperatören

A separate word like den is not usually needed with a simple noun on its own.

Compare:

• en tjej = a girl

• tjejen = the girl

• en mobiloperatör = a mobile operator
• mobiloperatören = the mobile operator

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

• den nya mobiloperatören = the new mobile operator

This is called double definiteness.

The basic form is tjej.

That is the indefinite singular form, and it usually goes with en:

• en tjej = a girl
• tjejen = the girl

A learner may also know flicka, which also means girl. The difference is mainly style and usage:

• tjej is very common in everyday Swedish
• flicka can sound more formal, literary, or more specifically like a younger girl depending on context

So tjejen is a very natural everyday word.

Väljer is the present tense of the verb välja, which means to choose.

So:

• välja = to choose
• väljer = chooses / is choosing

Examples:

• Jag väljer. = I choose / I am choosing.
• Tjejen väljer. = The girl chooses / is choosing.

Swedish present tense often covers both the English simple present and present continuous, depending on context.

Because Swedish does not usually make the same everyday distinction that English does between:

• chooses
• is choosing

The Swedish present tense often does both jobs.

So Tjejen väljer mobiloperatören can mean:

• The girl chooses the mobile operator
• The girl is choosing the mobile operator

Context tells you which is more natural.

If Swedish speakers want to make the ongoing sense especially clear, they can sometimes use other expressions, but the plain present tense is very common.

This is the normal word order for a basic statement:

subject + verb + object

• Tjejen = subject
• väljer = verb
• mobiloperatören = object

So literally:

• The girl
  • chooses
    • the mobile operator

This is very similar to English in a simple sentence.

Also, Swedish relies heavily on word order to show who does what, because nouns do not change form for subject and object the way they do in some other languages.

Mostly because of the word order.

In a normal Swedish statement, the noun before the verb is usually the subject, and the noun after the verb is usually the object.

So in:

• Tjejen väljer mobiloperatören

the usual reading is:

• Tjejen = the one doing the action
• mobiloperatören = the thing/person being chosen

Swedish nouns do not normally change form to mark subject vs. object, so position in the sentence matters a lot.

Yes. Swedish very often forms compound nouns by writing them as one word.

So:

• mobil = mobile
• operatör = operator
• mobiloperatör = mobile operator / mobile carrier

Then the definite ending is added to the whole compound:

• mobiloperatören = the mobile operator

This is extremely common in Swedish. English often writes similar expressions as two words, but Swedish usually combines them into one.

Also, the compound takes its grammatical gender from the last part. Since operatör is an en-word, mobiloperatör is also an en-word.

Yes, en mobiloperatör would also be possible, but it would mean something different.

• mobiloperatören = the mobile operator
• en mobiloperatör = a mobile operator

The sentence you have suggests a specific, known mobile operator. That could mean the speaker and listener already know which one is being talked about, or the context makes it specific.

If the meaning is more general, Swedish would often use the indefinite form:

• Tjejen väljer en mobiloperatör. = The girl is choosing a mobile operator.

So the definite form in your sentence is grammatically fine, but it sounds more specific.

A rough pronunciation guide is:

• Tjejen ≈ shay-yen
• väljer ≈ VEL-yer
• mobiloperatören ≈ mo-BEEL-op-er-a-TUR-en

A few useful notes:

• tj in tjej has a soft Swedish sound, often approximated as sh
• ä in väljer is somewhat like the e in bed
• j in Swedish is usually like English y
• ö in -tören does not exist in standard English; it is similar to German ö or French eu

The exact pronunciation varies a bit by dialect, but those rough English-style hints will get you close.

Yes. Even if the basic meaning is already clear, the Swedish can match more than one natural English version because of how the present tense works.

Possible translations include:

• The girl chooses the mobile operator.
• The girl is choosing the mobile operator.

If the context is about selecting a phone company, English might also use:

• The girl chooses the mobile carrier.
• The girl is choosing the phone provider.

So the exact best translation depends on context, even though the Swedish grammar stays the same.