Behandlingen starter i dag.

Because Norwegian usually puts the definite article on the end of the noun.

• behandling = treatment
• behandlingen = the treatment

So -en here is the definite ending, roughly the same as English the. In Bokmål, behandling is normally a common-gender noun, so the indefinite form is en behandling, and the definite singular form is behandlingen.

In Norwegian, you usually do not say a separate word for the in simple cases like this. The definiteness is already built into the noun:

• en behandling = a treatment
• behandlingen = the treatment

A separate determiner can appear in other structures, especially with adjectives, for example:

• den nye behandlingen = the new treatment

But in Behandlingen starter i dag, the ending -en already does the job.

Starter is the present tense form of the verb å starte.

• infinitive: å starte = to start
• present: starter = starts / is starting

Norwegian present tense is often made by adding -r to the infinitive:

• å komme → kommer
• å snakke → snakker
• å starte → starter

Unlike English, Norwegian verbs do not change for different persons:

• jeg starter
• du starter
• han/hun starter
• vi starter

It stays starter for all of them.

Yes. Norwegian often uses the present tense for something that begins in the near future, especially when it is planned, scheduled, or certain.

So Behandlingen starter i dag is perfectly natural for something that starts later today or from today onward.

This is very similar to English:

• The treatment starts today

If you want to emphasize the future more explicitly, you could also say:

• Behandlingen skal starte i dag

But the simple present is very common and natural here.

Yes. Behandlingen begynner i dag is also correct and natural.

Both starte and begynne can mean start/begin. In many situations, they are interchangeable.

Very roughly:

• starte can feel a little like start / get underway
• begynne is the more basic begin

In this sentence, either one works well.

Because the standard Norwegian expression for today is i dag, written as two words.

This is a fixed time expression, and it works like other common expressions:

• i dag = today
• i morgen = tomorrow
• i går = yesterday

The preposition i appears in several time expressions in Norwegian, even when it does not translate neatly word-for-word into English.

Yes, absolutely.

Both of these are correct:

• Behandlingen starter i dag
• I dag starter behandlingen

The difference is mainly emphasis.

• Behandlingen starter i dag = neutral statement
• I dag starter behandlingen = extra emphasis on today

This happens because Norwegian main clauses follow the verb-second rule. If you move i dag to the front, the verb still has to come second:

• I dag
  • starter
    • behandlingen

You would not normally say I dag behandlingen starter in a normal main clause.

You would usually say:

• En behandling starter i dag

That changes the noun from definite to indefinite:

• behandlingen = the treatment
• en behandling = a treatment

So the sentence you were given is talking about a specific treatment, not just any treatment.

A careful approximate pronunciation is:

beh-HAHN-dling-en STAR-ter ee dahg

A more IPA-like version is roughly:

/bəˈhɑn.dliŋ.ən ˈstɑʈ.ər i dɑːɡ/

A few notes:

• behandlingen has the stress on hand
• starter has the stress on the first syllable
• i dag is usually pronounced smoothly together in normal speech
• the exact r sound depends on dialect

If you are a beginner, a good practical target is:

beh-HAHN-dling-en STAR-ter ee DAHG

Not always. Behandling is a broader word that can mean things like:

• treatment
• handling
• processing
• consideration

But in a sentence like Behandlingen starter i dag, it will often be understood as treatment in a medical, therapeutic, or cosmetic sense, depending on context.

So the exact meaning comes from the situation, even though the grammar stays the same.