Avverto il cliente oggi.

Italian often leaves out subject pronouns because the verb ending already shows who is doing the action.
Here, avverto is the first-person singular form, so it already means I inform / I warn / I notify.

You could add io for emphasis or contrast:

• Io avverto il cliente oggi. = I’m the one informing the client today
• Io avverto il cliente, tu avverti il fornitore.

So the sentence without io is completely normal.

Avverto is the present indicative, first-person singular of avvertire.

So the pattern is:

• io avverto = I inform / I warn
• tu avverti
• lui/lei avverte
• noi avvertiamo
• voi avvertite
• loro avvertono

In this sentence, the present tense can refer to a present or near-future action, depending on context.

It can mean more than one thing. Avvertire is a flexible verb. Common meanings include:

• to inform / notify
• to warn
• to feel / perceive

The object usually helps you understand the meaning.

For example:

• Avverto il cliente. → usually I inform / notify / warn the client
• Avverto un dolore. → I feel a pain
• Avverto un odore strano. → I notice / perceive a strange smell

So with il cliente, the meaning is normally about communicating something to the client.

Because il cliente is the direct object of avverto.

In Italian, avvertire usually takes a direct object when it means inform / warn someone:

• Avverto il cliente
• Avvertiamo i clienti
• Hai avvertito Maria?

English learners sometimes expect to before the person, but Italian does not use a here.

Compare:

• Avverto il cliente = I inform/warn the client
• Dico al cliente = I say to the client

So avvertire works differently from verbs like dire.

Yes, in this sentence the definite article is normally required: il cliente.

Italian uses articles more consistently than English. With a singular countable noun like cliente, you usually need an article or some other determiner:

• il cliente = the client
• un cliente = a client
• questo cliente = this client
• mio cliente = my client

Just saying Avverto cliente would be incorrect in standard Italian.

Yes, the word order is flexible. Oggi can move depending on emphasis.

All of these are possible:

• Avverto il cliente oggi.
• Oggi avverto il cliente.
• Il cliente lo avverto oggi.
• Avverto oggi il cliente.

The most natural choice depends on what you want to emphasize:

• Oggi avverto il cliente emphasizes today
• Avverto il cliente oggi is a neutral statement
• Il cliente lo avverto oggi strongly highlights the client

Italian word order is usually more flexible than English, but some orders sound more natural than others.

Yes. Italian often uses the present tense for a planned or near-future action, especially when there is a time word like oggi.

So Avverto il cliente oggi can mean something like:

• I’m informing the client today
• I’ll inform the client today

The adverb oggi makes the time clear, so Italian does not always need a future form.

If you wanted, you could also say:

• Avvertirò il cliente oggi

But the present tense is very common and natural.

Often, yes. Avvisare is very common and often sounds more everyday in many contexts.

Compare:

• Avverto il cliente oggi
• Avviso il cliente oggi

Both can work, but there can be slight differences in tone depending on context.
Avvertire can sound a bit broader or sometimes slightly more formal, and it also has the extra meaning to perceive/feel, which avvisare does not have.

In many practical situations, though, native speakers would readily understand either one.

It can mean both, depending on context.

• in a legal, professional, or business relationship: client
• in a shop or service setting: customer

So il cliente is a very normal word, but the exact English translation depends on the situation.

You would normally use the direct object pronoun lo:

• Lo avverto oggi. = I’m informing/warning him/it today

If cliente refers to a woman, you would use la only if the noun itself is feminine, but cliente is usually grammatically masculine even when referring to a male or female client in many contexts. If you are referring specifically to a woman and want to make that clearer, context matters.

For the plural:

• Li avverto oggi. = I’m informing them today

A learner should mainly remember that because il cliente is a direct object, the pronoun is a direct object pronoun.

It is pronounced roughly like ah-VEHR-toh.

A few helpful points:

• the stress is on ver: avVÈRto
• the r is an Italian rolled or tapped r
• the final o is clearly pronounced

Also notice the double consonant in the infinitive avvertire. In careful pronunciation, the vv and rt are more distinct than in English-style pronunciation.

Yes, it can sound perfectly natural, especially in a practical context such as work, planning, or a conversation about tasks.

For example:

• Che fai oggi?
  Avverto il cliente oggi.

It is short and clear. In real conversation, though, speakers might often choose a slightly fuller version depending on context, such as:

• Oggi avverto il cliente.
• Avviso il cliente oggi.
• Oggi contatto il cliente.

But the original sentence is grammatically correct and natural.