Ho già messo la riunione di domani nel calendario.

Ho messo is the passato prossimo, a very common Italian past tense used for completed actions.

So Ho già messo la riunione di domani nel calendario means that the action of putting the meeting into the calendar is already done.

• ho = I have
• messo = past participle of mettere (to put)

English often uses either:

• I already put... in informal speech, or
• I’ve already put...

Italian strongly prefers the completed-past form here.

Già means already.

In compound tenses like ho messo, già usually goes between the auxiliary and the past participle:

• Ho già messo...

This is one of the most natural positions. You may also hear Già ho messo..., but that is much less common and usually more marked or emphatic.

So for a learner, ho già + past participle is a very useful pattern:

• Ho già visto
• Hai già fatto
• Abbiamo già prenotato

Because the auxiliary verb here is avere: ho messo.

With avere, the past participle usually does not agree with the direct object in normal sentences:

• Ho messo la riunione
• Ho visto la macchina
• Abbiamo comprato le sedie

So messo stays the same.

Agreement can happen in some cases with direct object pronouns before the verb, especially in careful writing:

• L’ho già messa nel calendario.

Here messa can agree with la riunione because la comes before the verb.

La riunione di domani literally means the meeting of tomorrow, but in natural English it corresponds to tomorrow’s meeting or the meeting tomorrow.

Italian often uses di + time expression to describe something associated with a day or time:

• la lezione di oggi = today’s lesson
• il treno di stasera = tonight’s train
• la partita di domenica = Sunday’s match

So di domani is a normal and very common way to say for tomorrow / tomorrow’s.

Sometimes, but they are not exactly the same.

• la riunione di domani = tomorrow’s meeting; this is the most natural phrase here.
• la riunione per domani can mean the meeting scheduled for tomorrow, but it can also sound a little more like intended for tomorrow.
• la riunione domani without di is generally less natural in standard Italian if you want it to directly modify riunione.

So in this sentence, la riunione di domani is the safest and most idiomatic choice.

Nel calendario is in + il = nel, meaning in the calendar.

Here, mettere qualcosa nel calendario means to place or enter something into a calendar, very much like adding an event to a calendar app or writing it in a planner.

Compare:

• mettere nel calendario = to put into the calendar
• essere in calendario = to be scheduled / to be on the calendar

So:

• Ho messo la riunione nel calendario = I put the meeting into the calendar
• La riunione è in calendario = The meeting is on the calendar / scheduled

The first focuses on the action of entering it; the second focuses on its status.

It can refer to either, depending on context.

In modern everyday use, calendario often means a digital calendar, especially if someone is talking about work, scheduling, phones, or email. But it can also mean a paper calendar or planner.

If you want to be more specific, Italian might also use:

• agenda = planner / diary / agenda book
• calendario del telefono = phone calendar
• calendario di Outlook / Google Calendar = specific digital calendar

So nel calendario is broad and flexible.

Italian uses definite articles more often than English.

• la riunione = the meeting
• il calendario = the calendar

In English, we might say I put tomorrow’s meeting in my calendar, where my is very common. In Italian, it is often perfectly natural to say simply nel calendario when the context already makes it clear whose calendar it is.

Italian does this a lot with things that are obvious from context:

• Ho preso la macchina = I took the car
• Ho controllato il telefono = I checked the phone
• L’ho scritto nell’agenda = I wrote it in the planner

Yes. Inserire is a very natural alternative, especially for entering data or adding something to a digital system.

Compare:

• Ho già messo... = I already put...
• Ho già inserito... = I already entered/added...

Mettere is very common and flexible in everyday speech.
Inserire can sound slightly more specific or a bit more formal/technical, especially for digital calendars, forms, or databases.

Other possible verbs:

• aggiungere = to add
• segnare = to note down / mark down

For example:

• Ho già aggiunto la riunione al calendario.
• Ho già segnato la riunione in agenda.

Yes, sometimes, but the nuance changes a little.

• mettere qualcosa nel calendario = put something in the calendar
• aggiungere qualcosa al calendario = add something to the calendar

So with mettere, nel calendario is the most natural match.
With aggiungere, al calendario is very natural:

• Ho già aggiunto la riunione di domani al calendario.

Using messo al calendario would sound less standard in this context.

You can replace it with la, because riunione is feminine singular:

• L’ho già messa nel calendario.

This means I’ve already put it in the calendar.

Notice two things:

1. la becomes l’ before ho
2. The past participle is often written as messa to agree with the feminine object pronoun

This is a very useful pattern:

• Hai prenotato la visita? → Sì, l’ho già prenotata.
• Hai scritto l’email? → Sì, l’ho già scritta.

The original order is very natural:

• Ho già messo la riunione di domani nel calendario.

But Italian word order is somewhat flexible, especially for emphasis.

Possible variations:

• Ho messo già la riunione di domani nel calendario.
  Understandable, but less natural than ho già messo
• La riunione di domani l’ho già messa nel calendario.
  This emphasizes the meeting tomorrow
• Nel calendario ho già messo la riunione di domani.
  This emphasizes where you put it

So the original version is the best neutral phrasing.