Il calciatore calcia il pallone.

Because there are two separate nouns, and each one needs its own article:

• il calciatore = the footballer / soccer player
• il pallone = the ball

Italian usually repeats the article where English would too. So Il calciatore calcia il pallone is literally structured as The footballer kicks the ball.

Italian definite articles change depending on the sound that follows.

Use il before most singular masculine nouns beginning with a normal consonant sound, like:

• il calciatore
• il pallone

Use lo before certain special sound combinations, such as:

• lo studente
• lo zaino
• lo psicologo

Since calciatore starts with a regular c sound, the correct article is il.

Yes. They come from the same word family.

• calciare = to kick
• calcia = he/she kicks
• calciatore = footballer / soccer player, literally someone connected with kicking or football

A learner may also notice calcio, which can mean football/soccer and can also mean kick in other contexts. These words are closely related.

Because calcia is the third-person singular present tense form of calciare.

Here is the relevant part of the conjugation:

• io calcio = I kick
• tu calci = you kick
• lui/lei calcia = he/she kicks

Since il calciatore is he in this sentence, Italian uses calcia.

A common point of confusion is that calcio is also a noun meaning football/soccer. So:

• io calcio = I kick
• il calcio = football/soccer

Same spelling, different grammar and meaning.

The key part is ci + a/o/u.

In Italian, cia, cio, and ciu are usually pronounced with a ch sound, and the i is not pronounced separately.

So:

• calcia sounds roughly like KAL-cha
• calciatore sounds roughly like kal-cha-TO-re

More carefully:

• calcia ≈ KAHL-chah
• calciatore ≈ kahl-chah-TOH-reh

The stress in calciatore falls on -to-.

Because not all masculine nouns end in -o.

Italian has several common masculine noun endings, including:

• -o: ragazzo
• -e: pallone actually ends in -e too, and it is masculine
• -ore: calciatore

So the final -e does not automatically make a noun feminine. Gender has to be learned with the noun, often together with its article:

• il calciatore
• il pallone

In this sentence, the usual Italian word order makes it clear:

• Il calciatore = subject
• calcia = verb
• il pallone = direct object

So the pattern is:

subject + verb + object

Italian does not mark subject and object with different articles the way some languages do. Both nouns use il because both are singular masculine nouns. Their role is understood mainly from word order and context.

Yes, but Il calciatore calcia il pallone is the most neutral and natural order.

Italian can move words around for emphasis, but that changes the feel of the sentence. For example:

• Il calciatore calcia il pallone = neutral
• Il pallone lo calcia il calciatore = marked, emphasizing the footballer as the one who kicks it

For a beginner, it is best to learn the standard pattern first:

subject + verb + object

Yes, but the meaning changes.

• Il calciatore calcia il pallone = the footballer kicks the ball
  This sounds like we are talking about a specific player and a specific ball, or simply describing a typical scene.
• Un calciatore calcia un pallone = a footballer kicks a ball
  This sounds more indefinite or generic.

So both are grammatical, but they do not mean exactly the same thing.

Both words exist, but they are not always used in exactly the same way.

• palla = ball, in a general sense
• pallone = large ball; often the football/soccer ball in sports contexts

In a football context, il pallone is very natural. So this sentence sounds like normal sports Italian.

No. Italian usually does not need subject pronouns unless you want emphasis or contrast.

So:

• Il calciatore calcia il pallone = perfectly normal
• Lui calcia il pallone = He kicks the ball
• Il calciatore lui calcia il pallone = possible only in special emphatic contexts, not the normal basic sentence

Italian verbs already show the person clearly, and here the noun il calciatore already tells us who is doing the action.