Acvariul are un filtru.

Because acvariul is the definite form, meaning the aquarium.

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

• acvariu = an aquarium / aquarium
• acvariul = the aquarium

Here, -l is the definite article ending added to acvariu.

In Romanian, the verb a avea means to have.
The form are means has or he/she/it has.

So:

• Acvariul are un filtru. = The aquarium has a filter.

This is different from English, where learners sometimes expect a form similar to is in simple descriptive sentences. Romanian uses are because the sentence expresses possession.

Un means a or an.

So:

• un filtru = a filter

Romanian indefinite articles change according to gender and number:

• un = masculine/neuter singular
• o = feminine singular
• plural usually has no direct equivalent of a/an

Since filtru is a masculine/neuter noun, it takes un.

This is the normal Romanian word order:

• Acvariul = subject
• are = verb
• un filtru = object

So it follows the common Subject + Verb + Object pattern, just like English:

• The aquarium has a filter.

Romanian word order can be flexible in some contexts, but this version is the most natural neutral order.

Both acvariu and filtru are typically treated as neuter nouns in Romanian.

A useful thing to know about Romanian neuter nouns is that they behave:

• like masculine in the singular
• like feminine in the plural

That is why in the singular you get:

• un acvariu
• un filtru

And in the definite singular:

• acvariul
• filtrul

The form acvariul is singular because it comes from acvariu.

If it were plural, you would see a different form, for example:

• acvariile = the aquariums

So:

• acvariul = the aquarium
• acvariile = the aquariums

Also, the verb are matches a singular subject here.

Because Romanian usually uses an enclitic definite article, meaning the article is attached to the end of the noun.

Compare:

• English: the aquarium
• Romanian: acvariul

This is one of the most important structural differences between Romanian and English.

A few more examples:

• băiat = boy

• băiatul = the boy

• filtru = filter
• filtrul = the filter

Yes, but it means something different.

• Acvariul are un filtru. = The aquarium has a filter.
• Acvariul are filtrul. = The aquarium has the filter.

The second one refers to a specific, already known filter. In most basic contexts, un filtru is the natural choice because you are just stating that there is a filter, not identifying a particular one.

A simple approximate pronunciation for an English speaker is:

ahk-vah-ree-ool AH-reh oon FEEL-troo

A few useful notes:

• c before v stays a hard k sound: ac-
• ă in are is a short central vowel, somewhat like the a in sofa
• u in un and filtru is pronounced, not silent
• Romanian words are usually pronounced quite consistently compared with English spelling

Yes. Are is the 3rd person singular form of a avea (to have).

Here is the present tense:

• eu am = I have
• tu ai = you have
• el/ea are = he/she has
• noi avem = we have
• voi aveți = you all have
• ei/ele au = they have

Since acvariul is a singular thing (it), Romanian uses are.

Normally, no, not in standard Romanian for this meaning.

• Acvariul are un filtru. = The aquarium has a filter.

If you say acvariu without the definite article, it usually would not work as a normal subject meaning the aquarium. Romanian generally needs the definite form when English would use the.

So in this sentence, acvariul is the correct natural form.