Domani partecipo alla gara di corsa con il mio amico.

In Italian, the subject pronoun is often omitted because the verb ending already shows who is doing the action.

• partecipo = I participate / I am participating
• The ending -o tells you the subject is io (I)

So Domani partecipo... is completely natural. You could say Io domani partecipo..., but that usually adds emphasis, such as I’m the one participating tomorrow.

Partecipo is in the present indicative.

Even though domani means tomorrow, Italian very often uses the present tense to talk about the near future when the time is clear from context.

So:

• Domani partecipo alla gara = Tomorrow I’m participating in the race

This is very common and natural. You could also use a future form:

• Domani parteciperò alla gara

Both are correct. The present often sounds a bit more immediate or conversational.

Because the verb partecipare normally takes the preposition a.

• partecipare a qualcosa = to participate in something

So:

• a + la gara → alla gara

This is a contraction:

• a + la = alla

Examples:

• Partecipo al torneo.
• Partecipiamo alla festa.
• Hai partecipato al concorso?

So after partecipare, you should expect a.

Alla is simply the combination of:

• a = to / in
• la = the (feminine singular)

Together:

• alla gara = in the race / to the race in the sense required by partecipare

Italian often combines prepositions and articles into one word:

• a + il = al
• a + lo = allo
• a + la = alla
• a + i = ai
• a + gli = agli
• a + le = alle

Here di links the type of race to the activity.

• gara = competition / race
• corsa = running
• gara di corsa = running race

So di here is like of or sometimes natural English for/in depending on context.

Similar examples:

• gara di nuoto = swimming race/competition
• scarpe da corsa = running shoes
• corso di italiano = Italian course

In this sentence, gara di corsa means a race whose activity is running.

Yes, often you could, depending on context.

• Domani partecipo a una corsa = Tomorrow I’m taking part in a race/run

But gara di corsa is a bit more explicit: it clearly says it is a competition involving running.

Roughly:

• corsa can mean run, race, or running
• gara di corsa stresses the competitive event more clearly

So the original sentence is perfectly understandable and natural, especially for learners because it is very clear.

In Italian, possessive adjectives usually come with the definite article:

• il mio amico = my friend
• la mia macchina = my car
• i miei libri = my books

So con il mio amico is the normal form.

A useful note: With singular family members, Italian often drops the article:

• mia madre
• mio fratello
• tuo padre

But amico is not a family member, so you keep the article:

• il mio amico

Because amico is a masculine singular noun, so the possessive must agree with it.

• amico → masculine singular
• therefore mio

Compare:

• il mio amico = my male friend
• la mia amica = my female friend

The possessive agrees with the noun possessed, not with the speaker.

Yes. Italian word order is flexible, and domani can move depending on emphasis.

All of these are possible:

• Domani partecipo alla gara di corsa con il mio amico.
• Partecipo domani alla gara di corsa con il mio amico.
• Partecipo alla gara di corsa domani con il mio amico.

The version with Domani at the beginning is very natural because it sets the time immediately.

Grammatically, con il mio amico just means with my friend. By itself, it does not fully specify whether:

• you are both participating together, or
• your friend is simply accompanying you

In many real situations, people may assume he is participating with you, but the phrase itself is not completely precise about that.

If you want to be clearer, you could say:

• Domani io e il mio amico partecipiamo alla gara di corsa.
  = Tomorrow my friend and I are participating in the running race.

It is pronounced roughly par-te-CHEE-po, with the stress on ci:

• par-te-ci-po

In standard Italian, ci before i/o/a sounds like ch in cheese.

So:

• partecipo ≈ par-teh-CHEE-poh

This can help distinguish it from English-style pronunciation guesses like par-teh-see-po, which would not be correct in Italian.