Animalul de companie doarme aici.

Because Romanian usually puts the definite article at the end of the noun.

• animal = animal
• animalul = the animal

So instead of a separate word like the, Romanian adds a suffix to the noun.

Animal is a neuter noun in Romanian, and in the singular neuter nouns usually behave like masculine nouns.

For a noun ending in a consonant, the definite singular ending is often -ul:

• băiat → băiatul
• copil → copilul
• animal → animalul

So animalul is the normal definite singular form.

De companie is the Romanian expression used for pet.

Literally, de often means of or for, and companie means company/companionship. So the phrase suggests an animal kept for companionship.

In Romanian, this type of modifier usually comes after the noun:

• animal de companie = pet / companion animal

You cannot normally say companie animal or animal companie.

The difference is indefinite vs. definite:

• animal de companie = a pet / pet animal
• animalul de companie = the pet

Only the main noun gets the definite ending. The phrase de companie stays the same.

Because doarme is the 3rd person singular present form of a dormi = to sleep.

Here the subject is singular:

• animalul de companie = one pet

So the verb must also be singular:

• animalul de companie doarme

A useful mini-pattern:

• eu dorm
• tu dormi
• el/ea doarme
• noi dormim
• voi dormiți
• ei/ele dorm

So doarme matches he/she/it sleeps.

Because Romanian does not need a subject pronoun here.

The subject is already stated:

• Animalul de companie = the subject

So adding a pronoun would be unnecessary. Romanian often leaves subject pronouns out unless they are needed for emphasis or contrast.

In fact, if the subject were already clear from context, Romanian could even say just:

• Doarme aici. = Sleeps here / It sleeps here

The basic, neutral order here is:

• Subject + Verb + Adverb
• Animalul de companie doarme aici.

That is the safest order for a learner.

But Romanian word order is fairly flexible. For example:

• Aici doarme animalul de companie. = emphasis on here

This is possible, but the original version sounds more neutral and straightforward.

Aici is an adverb of place, meaning here.

It does not change form for gender, number, or case. It simply tells you where the action happens.

Examples:

• Doarme aici. = Sleeps here.
• Aici doarme. = Here sleeps / It sleeps here.

It would be:

• Animalele de companie dorm aici.

Changes:

• animalul → animalele = the pets
• doarme → dorm = sleep

This is also a good example of how Romanian neuter nouns behave:

• in the singular, they act like masculine
• in the plural, they act like feminine

Yes, pet is also used in modern Romanian, especially in informal speech.

So you may hear things like:

• Petul doarme aici.

But animal de companie is a more traditional and fully Romanian expression, and it is very common in neutral or formal language.

In everyday speech, people may also be even more specific and just say:

• Câinele doarme aici. = The dog sleeps here.
• Pisica doarme aici. = The cat sleeps here.