Il partito cambia posizione oggi.

Because partito is a singular countable noun, and in Italian you usually need an article with it when you mean the party in a general sentence.

• il partito = the party
• un partito = a party

Leaving the article out would normally sound incomplete here.

Partito is a masculine singular noun, and it begins with a consonant sound, so the normal definite article is il.

Compare:

• il partito
• il libro
• il ministro

You would not use la because that is for feminine singular nouns.

Because cambia is the conjugated form of the verb, while cambiare is the infinitive.

Here the subject is il partito = the party, which is third person singular, so:

• cambiare = to change
• il partito cambia = the party changes / is changing

This is the normal present-tense form.

It is the present indicative.

For cambiare, the present forms are:

• io cambio
• tu cambi
• lui/lei cambia
• noi cambiamo
• voi cambiate
• loro cambiano

So cambia matches il partito.

Italian often omits the article in expressions like cambiare posizione, where the noun is part of a fairly direct object phrase.

So:

• cambiare posizione = to change position / stance

This works a bit like other verb + noun combinations where English might also avoid a very specific article depending on context.

That said, cambiare la posizione is also possible, but it usually sounds more specific, as if you mean a particular position already identified.

In this sentence, it most naturally means position in the sense of a stance, view, or political line, not physical location.

Because the subject is il partito, a political context is the most natural reading:

• the party changes its position
• the party changes stance

Italian often uses posizione this way.

Yes, partito can also be the past participle of partire meaning left or departed, but not in this sentence.

Here it is clearly a noun because:

• it is preceded by il
• it functions as the subject of the sentence

So here il partito can only mean the party.

Yes, oggi can move. Italian word order is flexible.

These are all possible:

• Il partito cambia posizione oggi.
• Oggi il partito cambia posizione.
• Il partito oggi cambia posizione.

The version with oggi at the end is perfectly natural and neutral. Putting oggi first often gives it a little more emphasis.

Yes. The Italian present tense can often correspond to several English forms depending on context:

• The party changes position today
• The party is changing position today
• sometimes even The party will change position today

Italian uses the simple present more broadly than English does.

Yes, it is natural and understandable, especially in political or formal contexts.

It can mean:

• to change position
• to shift stance
• to change one’s line

Depending on context, Italians might also say:

• cambiare idea = change one’s mind
• cambiare atteggiamento = change attitude
• cambiare linea = change line/policy
• modificare la propria posizione = modify one’s position

But cambiare posizione is perfectly fine.

It is pronounced roughly KAHM-byah.

A useful point: in cambia, the ia after b helps create the -bya sound.

So:

• cam- sounds like kahm
• -bia sounds like bya

The stress is on the first syllable: CAM-bia.

Approximate pronunciations:

• posizione ≈ poh-zee-TSYOH-neh
• oggi ≈ OD-jee

A couple of details:

• In posizione, the zi sound is like tsy / dz depending on accent, but poh-zee-TSYOH-neh is a good learner approximation.
• In oggi, the gg before i sounds like the j in judge, but held a bit more firmly.

Not this subject, because the subject is a noun phrase: il partito.

Italian often drops subject pronouns like io, tu, lui, etc., because the verb ending already shows the person. But that does not mean you normally drop a full noun subject whenever you want.

So:

• Cambia posizione oggi is grammatically possible, but now it means He/She/It changes position today or (someone/something) changes position today, and the subject would have to be understood from context.
• If you specifically want the party, keeping il partito is the normal choice.

You would make both the noun and the verb plural:

• I partiti cambiano posizione oggi.

Changes:

• il → i
• partito → partiti
• cambia → cambiano

So the sentence agrees in number all the way through.