El actor estadounidense ganó el premio.

Because both actor and premio are masculine singular nouns, so they take the masculine singular definite article el.

• el actor = the actor
• el premio = the prize / the award

In Spanish, articles have to agree with the noun in gender and number.

• el = masculine singular
• la = feminine singular
• los = masculine plural
• las = feminine plural

So if the noun changed, the article would change too:

• la actriz = the actress
• los actores = the actors

Estadounidense specifically means from the United States. It is the most precise and standard choice for American in this sense.

Although americano is sometimes used in everyday speech, it can also mean from the Americas more generally, not just the U.S. Because of that, estadounidense is often preferred when you want to be clear.

So:

• actor estadounidense = U.S. actor / American actor
• actor americano = possible in some contexts, but less precise

Because in Spanish, adjectives usually come after the noun.

So:

• el actor estadounidense literally follows the pattern the actor American

This is the normal word order in Spanish for many descriptive adjectives, especially ones of nationality.

More examples:

• el coche español = the Spanish car
• la cantante mexicana = the Mexican singer

Nationality adjectives very commonly come after the noun.

In the singular, estadounidense stays the same for both masculine and feminine.

• el actor estadounidense
• la actriz estadounidense

So unlike adjectives such as español / española, this one does not change its ending for gender in the singular.

In the plural, it adds -s:

• los actores estadounidenses
• las actrices estadounidenses

The accent mark in ganó is very important because it shows both pronunciation and meaning.

• ganó = he/she/it won (preterite, 3rd person singular)
• gano = I win / I do win (present, 1st person singular)

So the accent helps distinguish:

• Yo gano = I win
• Él ganó = He won

Without the accent, it would be a different verb form.

Ganó is the preterite tense: the simple past used for a completed action.

Here it means:

• he won or
• the actor won

It presents the action as finished.

Some related forms of ganar:

• gano = I win / I am winning
• ganaba = I was winning / used to win
• gané = I won
• ganó = he/she won

In this sentence, the preterite is natural because winning the prize is seen as a completed event.

Because Spanish usually uses an article where English might not omit it.

To refer to the actor, Spanish normally says:

• El actor estadounidense...

Leaving out el would usually sound unnatural here unless you were using a very special style, such as a headline or note.

So in normal sentences, Spanish often requires the article before a noun when English might also use the, and sometimes even when English would leave it out.

Traditionally:

• actor = actor
• actriz = actress

So if the person is female, many speakers would normally say:

• La actriz estadounidense ganó el premio.

However, in some modern contexts, actor may sometimes be used more broadly, but for learners, the clearest standard distinction is:

• actor for a man
• actriz for a woman

In standard Peninsular Spanish, it is pronounced approximately like:

es-ta-do-u-ni-DEN-se

A few useful notes:

• The stress is on -den-
• The d between vowels is softer than English d
• In much of Spain, the c/z sound is pronounced as a th sound, but that does not affect this word because it has s, not z or c before e/i

A rough English-friendly guide: ehs-tah-doh-oo-nee-DEN-seh

El actor americano ganó el premio is possible, but estadounidense is usually more precise for from the United States.

El estadounidense actor is not the normal word order. In Spanish, nationality adjectives usually come after the noun, so the natural version is:

• El actor estadounidense...

Putting the adjective before the noun would sound unusual and generally not appropriate here.

Because there is no direct object that is a person.

The structure is:

• El actor estadounidense = subject
• ganó = verb
• el premio = direct object

The personal a is used before a specific human direct object, for example:

• Vi al actor. = I saw the actor.

But here the direct object is el premio, which is a thing, not a person, so there is no personal a.

Yes. Premio can mean both prize and award, depending on context.

For example:

• ganó un premio = won a prize / won an award
• premio literario = literary award
• premio en metálico = cash prize

So in this sentence, el premio could be translated as either the prize or the award, depending on the situation.