Aviso l'agència immobiliària.

Aviso is the 1st person singular present indicative of the verb avisar.

So:

• avisar = to notify / to inform / to warn / to let someone know
• aviso = I notify / I inform / I warn / I let ... know

In this sentence, it means I am doing the action.

Catalan usually drops subject pronouns when they are clear from the verb ending.

So instead of saying:

• Jo aviso l'agència immobiliària

it is very natural to say:

• Aviso l'agència immobiliària

The ending -o already shows the subject is I.

You can add jo if you want emphasis or contrast:

• Jo aviso l'agència immobiliària, no tu.
  I notify the real estate agency, not you.

Because the feminine singular definite article la becomes l' before a word beginning with a vowel or mute h.

So:

• la + agència → l'agència

This is called elision.

Compare:

• la casa
• l'agència
• l'escola
• l'habitació

Catalan often uses the definite article when talking about a specific, known thing.

So l'agència immobiliària means the real estate agency.

If you meant an unspecified one, you would normally use:

• una agència immobiliària = a real estate agency

So the sentence suggests the speaker has a particular agency in mind.

Immobiliària is an adjective modifying agència.

• agència = agency
• immobiliària = real-estate / property-related

It agrees with agència because agència is:

• feminine
• singular

So the adjective is also:

• feminine singular → immobiliària

Compare:

• agència immobiliària = real estate agency
• agent immobiliari = real estate agent (masculine singular)

Yes. This is a normal Catalan word order.

The full structure is basically:

• (Jo) aviso l'agència immobiliària.
• Subject + verb + direct object

The subject jo is omitted, so what you see is:

• Verb + direct object

That is completely natural in Catalan because the verb form already tells you the subject.

Yes. Depending on context, avisar can correspond to several English verbs, such as:

• to notify
• to inform
• to warn
• to let someone know
• sometimes even to call/contact in certain contexts

So aviso l'agència immobiliària could be understood slightly differently depending on the situation, even though the grammar stays the same.

Catalan simple present can often cover both:

• I notify / I inform
• I am notifying / I’m informing

The exact English translation depends on context.

So aviso l'agència immobiliària could describe:

• a habitual action
• a present action
• sometimes even a near-future action, depending on context

Catalan uses the simple present more broadly than English sometimes does.

You would say:

• Jo aviso l'agència immobiliària.

This is grammatically correct, but it is usually used only when you want:

• emphasis
• contrast
• clarity

In ordinary neutral speech, Aviso l'agència immobiliària is more natural.

The stress is on the è in agència:

• a-GÈN-ci-a

The written accent mark tells you where the stress goes.

And in immobiliària, the stress is on à:

• im-mo-bi-li-À-ri-a

Those accent marks are important in Catalan because they help show pronunciation and stress.