La guía enseñó un boceto antiguo antes de hablar del lienzo final.

Because guía here refers to a female guide, so the article is la.

• el guía = the male guide
• la guía = the female guide

The noun guía can be masculine or feminine depending on the person. Even though it ends in -a, it can still be masculine when referring to a man.

It can also mean guidebook in other contexts, but in this sentence la guía clearly means the guide (a woman).

Enseñó is the 3rd person singular preterite of enseñar.

So:

• enseñar = to show / to teach
• enseñó = he/she showed / taught

In this sentence, enseñar means to show, not to teach.

The preterite is used because the sentence describes a completed action in the past:

• La guía enseñó un boceto antiguo... = The guide showed an old sketch...

The accent mark is crucial.

• enseño = I teach/show
• enseñó = he/she showed

So:

• yo enseño = I show / I teach
• él/ella enseñó = he/she showed

In writing, that accent changes both the pronunciation and the meaning.

Because un boceto antiguo is a thing, not a person.

In Spanish, the personal a is usually used before a specific human direct object:

• Veo a María.
• La guía saludó a los turistas.

But with objects or things, you normally do not use a:

• La guía enseñó un boceto antiguo.

Since boceto is a sketch, not a person, there is no personal a.

Both are possible, but they can sound slightly different.

In the given sentence:

• un boceto antiguo = an old/ancient sketch

Here, antiguo after the noun is the most straightforward, literal description: the sketch is old.

If you put the adjective before the noun:

• un antiguo boceto

this can sometimes sound more literary or can suggest former in other contexts, depending on the adjective and sentence. With antiguo, position can matter:

• un amigo antiguo = an old friend in age or from long ago, depending on context
• un antiguo amigo = a former friend / a long-standing old friend, depending on context

For boceto, un boceto antiguo is the most neutral and natural way to say the sketch was old.

Because after antes de, Spanish uses an infinitive when the subject stays general or is the same as the main clause.

• antes de hablar = before speaking / before talking

The structure is:

• antes de + infinitive

Examples:

• antes de comer = before eating
• antes de salir = before leaving
• antes de hablar del lienzo final = before talking about the final canvas

You cannot say antes hablar here.

Because de + el contracts to del.

So:

• de + el = del
• a + el = al

That is why:

• hablar del lienzo final = to talk about the final canvas

Not:

• hablar de el lienzo final

This contraction is mandatory in normal Spanish, except when el is part of a proper name:

• Vengo de El Escorial.
  Here it stays separate because El Escorial is a place name.

Hablar de means to talk about.

So:

• hablar del lienzo final = to talk about the final canvas

This is a very common pattern:

• hablar de arte = to talk about art
• hablar del museo = to talk about the museum
• hablar de un pintor famoso = to talk about a famous painter

English speakers often need to remember that Spanish uses de after hablar when it means talk about.

In Spanish, adjectives usually come after the noun.

So the normal order is:

• lienzo final = final canvas
• boceto antiguo = old sketch

Putting final before the noun is much less neutral and may sound poetic, emphatic, or unusual depending on context. The basic learner rule is:

• noun + adjective = most common order

Lienzo literally means canvas, especially in an artistic context.

In this sentence, it likely refers to the finished painted canvas or final artwork.

It can also sometimes mean the fabric itself, but here, because of boceto and final, the art meaning is the natural one:

• boceto = sketch
• lienzo final = final canvas / final painting on canvas

Grammatically, yes, because enseñar can mean both to teach and to show. But in this sentence, showed is the natural meaning.

Why? Because the object is un boceto antiguo:

• La guía enseñó un boceto antiguo...

You show a sketch. You do not usually teach a sketch.

So context tells you the correct meaning.

Because hablar is an infinitive, and Spanish often leaves the subject implied when it is understood from context.

The sentence means:

• The guide showed an old sketch before talking about the final canvas.

Spanish does not need to say:

• antes de que ella hablara...

unless you want a different structure or a different subject.

Compare:

• antes de hablar del lienzo final = before talking about the final canvas
• antes de que hablara del lienzo final = before she talked about the final canvas

The version with the infinitive is shorter and very common.

Yes. Mostró would also work very well.

• enseñó = showed
• mostró = showed

In many contexts, mostrar is a very direct equivalent of to show:

• La guía mostró un boceto antiguo...

Enseñar is also very common in everyday Spanish for showing something to someone.

So both are possible, though some learners first meet enseñar only as to teach, which can be confusing.

Hablar is not a tense here; it is the infinitive.

The only fully conjugated verb in the sentence is:

• enseñó = showed

The phrase:

• antes de hablar del lienzo final

uses hablar in the infinitive after antes de.

So the sentence has:

• one main past action: enseñó
• one subordinate infinitive phrase: antes de hablar...

Most likely it means the finished/final version.

Because the sentence contrasts:

• un boceto antiguo = an old sketch
• el lienzo final = the final canvas

This creates a natural sequence in an art context:

1. an earlier sketch
2. the final artwork

So final here probably means the completed version, not just the last canvas in a row.