Ella armó el escenario ayer.

Yes. In Spanish, subject pronouns are often omitted because the verb ending already shows who is doing the action.

So both of these are fine:

• Ella armó el escenario ayer.
• Armó el escenario ayer.

Including ella can add emphasis, contrast, or clarity, for example if you want to stress that she did it, not someone else.

Armó is the preterite form of armar, used for a completed action in the past.

• arma = she assembles / she is assembling / you formal assemble
• armó = she assembled / she set up

In this sentence, ayer tells you the action happened yesterday, so Spanish uses the past form: armó.

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

• armo = I assemble / I set up
• armó = she assembled / he assembled / you formal assembled

So the accent is important. Without it, the form would mean something different.

It is the pretérito indefinido in Spain, often just called the preterite in English grammar explanations.

It is used for a finished action at a specific time in the past. Since ayer refers to a completed time period, the preterite fits naturally:

• ayer armó... = she set it up yesterday

Spanish usually uses an article where English sometimes does not.

So Spanish prefers:

• armó el escenario

rather than:

• armó escenario ❌

Also, escenario is a masculine singular noun, so it takes el.

Yes. Escenario is masculine: el escenario.

And yes, nouns ending in -o are very often masculine, though there are exceptions. Here it follows the common pattern.

If you used an adjective, it would also agree in masculine singular:

• el escenario grande
• el escenario principal

Because el escenario is a direct object, but it is a thing, not a person.

Spanish uses the personal a mainly with specific people (and sometimes pets or personified beings):

• Veo a María.
• Conozco al actor.

But with things, you normally do not use a:

• Armó el escenario.

Yes. Spanish word order is flexible, especially with time expressions like ayer.

All of these are possible:

• Ella armó el escenario ayer.
• Ayer, ella armó el escenario.
• Ella ayer armó el escenario.

The first one is very neutral and natural. Moving ayer can change the emphasis slightly, but not the basic meaning.

It is understandable, but in Spain, montar is often more common for physically setting up something like a stage.

So in Spain you may also hear:

• Ella montó el escenario ayer.

That said, armó is still grammatical and clear. Depending on context, armar can sound like assemble, put together, or set up.

No. In this sentence, ella is the subject pronoun meaning she.

Be careful not to confuse it with:

• ella = she
• la = her / it
• el = the
• él = he

The accents and spelling matter a lot in Spanish.

Yes. In Spanish, yes/no questions often keep the same word order. You mainly show the question with intonation in speech or with question marks in writing:

• ¿Ella armó el escenario ayer?

This means Did she set up the stage yesterday?

You could also omit the pronoun:

• ¿Armó el escenario ayer?

That can mean Did she/he/you formal set up the stage yesterday? The wider context tells you who is meant.