Veterinären behandlar infektionen.

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

• veterinär = veterinarian / vet
• veterinären = the veterinarian / the vet
• infektion = infection
• infektionen = the infection

So the -en ending here is the definite form, roughly equivalent to English the.

In simple Swedish noun phrases, definiteness is often shown by a suffix attached to the noun.

So instead of:

• English: the veterinarian
• Swedish: veterinären

And instead of:

• English: the infection
• Swedish: infektionen

A separate definite word can appear in other structures, especially with adjectives, but not in this sentence.

Behandlar is the present tense of the verb behandla, meaning to treat.

• att behandla = to treat
• behandlar = treats / is treating

In Swedish, the present tense often ends in -r.

Examples:

• Jag behandlar = I treat / am treating
• Veterinären behandlar = The veterinarian treats / is treating

It can mean both, depending on context.

Swedish present tense often covers both:

• a general present: treats
• an ongoing action: is treating

So Veterinären behandlar infektionen could mean:

• The veterinarian treats the infection
• The veterinarian is treating the infection

Usually the situation makes the intended meaning clear.

Yes, in this sentence it is a straightforward subject–verb–object pattern:

• Veterinären = subject
• behandlar = verb
• infektionen = object

So it matches English word order:

• The veterinarian
  • treats
    • the infection

This makes the sentence relatively easy for English speakers.

In this sentence, word order tells you that:

• Veterinären comes first, so it is the subject
• infektionen comes after the verb, so it is the object

Swedish usually relies heavily on word order in simple statements.

So:

• Veterinären behandlar infektionen = The veterinarian treats the infection

If you reversed the nouns, the meaning would change:

• Infektionen behandlar veterinären would mean The infection treats the veterinarian, which is grammatically possible but semantically odd.

Their base forms are:

• veterinär → veterinären
• infektion → infektionen

The base form is the indefinite singular form, the form you usually see in dictionaries.

So if you look these up, you would search for:

• veterinär
• infektion

Both veterinär and infektion are common gender nouns in Swedish.

Swedish has two grammatical genders:

• common gender (en-words)
• neuter (ett-words)

These are en-words, which is why their definite singular form ends in -en:

• en veterinär → veterinären
• en infektion → infektionen

Yes, gender matters because it affects articles and noun endings.

Yes, that is the fully indefinite version.

Compare:

• Veterinären behandlar infektionen = The veterinarian treats the infection
• En veterinär behandlar en infektion = A veterinarian treats an infection

So Swedish uses:

• en for the indefinite article (a/an)
• a suffix like -en for the definite article (the)

A careful approximate pronunciation is:

veh-teh-ri-NAIR-en behan-dlar in-fek-SHOO-nen

A few helpful notes:

• ä in veterinären is somewhat like the vowel in English air, though not exactly the same.
• behandlar has stress on the second syllable: be-HAND-lar
• tion in infektion is pronounced like shoon: infek-SHOON
• r sounds in Swedish may be different from English depending on accent.

If you are learning pronunciation, it is best to listen to native audio, since Swedish pitch accent and vowel quality are important.

Yes. Veterinär is the full word for veterinarian, but in natural English, vet is often the most idiomatic translation.

So:

• Veterinären behandlar infektionen could be translated as
  • The veterinarian is treating the infection
  • The vet is treating the infection

Which one sounds best depends on the style and context.

Yes, absolutely. It is a normal, complete Swedish sentence.

It has everything needed:

• a subject: Veterinären
• a verb: behandlar
• an object: infektionen

It sounds like a clear statement and would be perfectly natural in a medical or veterinary context.