La actuación terminó tarde.

Because Spanish usually needs an article before a singular count noun used as the subject of a sentence.

So la actuación means the performance / the act / the appearance. In English, we sometimes omit the in places where Spanish would not.

• La actuación terminó tarde.
• More natural than: Actuación terminó tarde ❌

Actuación is a feminine noun, so it takes la.

A very useful pattern is that many nouns ending in -ción are feminine:

• la canción
• la información
• la situación
• la actuación

So even if you do not know the word yet, -ción is a strong clue that the noun is feminine.

In this sentence, actuación refers to a performance, act, stage appearance, or sometimes a performer’s turn.

Depending on context, it could mean things like:

• a musical performance
• a comedy act
• a dance performance
• an actor’s performance

It does not usually mean action in the English sense of doing something. Even though it looks a bit like action, the meaning is different.

Terminó is the preterite form of terminar, in the third person singular:

• yo terminé
• tú terminaste
• él/ella/usted terminó

The preterite is used for a completed action in the past. Here, the performance ended at a specific point in time.

Because terminó presents the ending as a completed event.

• terminó = ended / came to an end
• terminaba = was ending / used to end / would end, depending on context

In this sentence, we are talking about one finished event, so terminó is the natural choice.

Compare:

• La actuación terminó tarde.
  The performance ended late.

• La actuación terminaba tarde todos los días.
  The performance used to end late every day.

The accent mark shows the stress: ter-mi-NÓ.

It also helps distinguish this form from termino:

• terminó = he/she/it ended
• termino = I end / term (depending on context)

So the written accent is important both for pronunciation and meaning.

Here, tarde is an adverb, meaning late.

It modifies the verb terminó, telling us when the performance ended.

• terminó tarde = ended late

This is different from an adjective, which would describe a noun.

Yes. Tarde can be:

1. a noun: the afternoon

   • la tarde

2. an adverb: late

   • Llegó tarde.
   • La actuación terminó tarde.

In your sentence, there is no article before tarde, and it is describing the verb terminó, so it means late, not afternoon.

Yes. Acabó is a very natural alternative.

• La actuación terminó tarde.
• La actuación acabó tarde.

Both are correct and common. In many contexts they are almost interchangeable.

A rough difference:

• terminar = to end, to finish
• acabar = to finish, to come to an end

In everyday speech, either works well here.

In standard Peninsular Spanish, it is pronounced approximately:

ac-tua-CIÓN

More specifically:

• ción is pronounced with the th sound of think in most of Spain: -θión
• so the full word is roughly ak-twa-THYON

A simple learner-friendly version:

• ac as in actor
• tua like twa
• ción like thyón in Spain

Also remember that the stress falls on the last syllable: actuaCIÓN.

Yes, if the context already makes clear what ended.

Spanish often drops information that is already understood. So:

• La actuación terminó tarde. = explicit
• Terminó tarde. = natural if everyone knows you are talking about the performance

But if you want to state the subject clearly, keeping La actuación is the best choice.

Yes, but the basic order here is the most neutral:

• La actuación terminó tarde.

You may also hear something like:

• Terminó tarde la actuación.

That is possible, but it sounds more marked or stylistic, and it is less neutral for a beginner. The original sentence is the most straightforward version to learn first.