El acto empezó temprano.

Because acto is a masculine noun in Spanish, so it takes the masculine singular article el.

• el acto = the event / the act / the ceremony
• la is only used with feminine singular nouns

This is something you usually just have to learn with the noun.

Sometimes, but not always. In this sentence, acto often means something like an event, ceremony, official function, or public gathering, depending on context.

It can also mean act in other contexts, but Spanish acto is commonly used in situations where English might prefer event rather than act.

So a learner should not assume it always matches the English word act exactly.

Because empezó is in the preterite past tense, meaning the action happened in the past and is seen as completed.

• empieza = starts / is starting
• empezó = started

So El acto empezó temprano refers to a past event that began early.

The accent mark shows where the stress goes and also helps distinguish the form.

• empezo would be incorrect
• empezó is the third-person singular preterite form of empezar

So the accent is not optional. It is part of the correct spelling.

Because the sentence presents the beginning of the event as a completed action, which fits the preterite: empezó.

• empezó = it started
• empezaba = it was starting / it used to start

A native English speaker often has trouble with this difference. Here, the sentence is simply telling us that the event started early, so empezó is the natural choice.

Because temprano can function as an adverb in Spanish, meaning early.

So:

• empezó temprano = started early

Spanish often uses adjective forms adverbially in certain common expressions, and temprano is one of the most common examples.
Tempranamente exists, but it sounds much less natural here and is not what speakers would normally say.

No, because here temprano is not describing the noun acto. It is describing how the action happened — in other words, it modifies the verb empezó.

That makes it an adverb, and adverbs do not agree in gender or number.

Compare:

• un acto temprano = an early event → temprano is an adjective
• el acto empezó temprano = the event started early → temprano is an adverb

Yes. Comenzó is a very natural alternative to empezó.

• El acto empezó temprano
• El acto comenzó temprano

Both are correct and common. In many situations, they mean essentially the same thing.
Empezar is often slightly more common in everyday speech, but comenzar is also very standard.

Normally, no. In Spanish, a singular countable noun like acto usually needs an article or another determiner.

So:

• El acto empezó temprano = correct
• Acto empezó temprano = generally incorrect

English often allows the to be omitted in headlines or certain styles, but standard Spanish normally keeps the article here.

You could say Empezó temprano el acto, and it is grammatically possible, but El acto empezó temprano is the most neutral and natural order.

Spanish word order is more flexible than English, but changing it can affect emphasis or style.

• El acto empezó temprano = neutral statement
• Empezó temprano el acto = more marked, perhaps for emphasis or in a literary/context-driven style

For most learners, the original order is the safest choice.