La profesora dejó los folios sobre la mesa y empezó la clase.

Because Spanish usually uses an article with a specific noun when you mean the teacher, the table, the papers, etc.

So:

• la profesora = the teacher
• profesora on its own is less natural here

Also, profesora is the feminine form. A male teacher would be el profesor.

A useful contrast:

• Es profesora. = She is a teacher.
• La profesora empezó la clase. = The teacher started the class.

In the first sentence, the profession is being stated, so no article is normally used. In the second, you are talking about a specific person.

Yes, dejó comes from dejar, which often means to leave. But in this sentence it means something like:

• left
• set down
• put down

So dejó los folios sobre la mesa means she placed the papers on the table.

It does not mean she left the room or abandoned the papers. Context tells you it means put/left them there.

They are in the preterite tense: dejó and empezó.

The preterite is used for completed actions, especially in a sequence:

1. she put the papers on the table
2. she started the class

That is exactly the kind of situation where Spanish normally uses the preterite.

If you used the imperfect instead:

• dejaba
• empezaba

it would sound more like background, description, or repeated/habitual action in the past.

The accent marks show the correct stress and also help distinguish these forms from other verb forms.

• dejó = he/she left / you left
• dejo = I leave

And:

• empezó = he/she started / you started

The written accent tells you the stress falls on the last syllable:

• de-JÓ
• em-pe-ZÓ

So the accent is an important part of the spelling.

Because empezar is a stem-changing verb in the present, but not in this preterite form.

Compare:

Present

• empiezo
• empieza

Preterite

• empecé
• empezó

So the e → ie stem change does not happen in empezó.

This is a very common point for learners: a verb can stem-change in the present but not in the preterite.

Not really. In Spain, folios usually means sheets of paper, often ordinary loose pages or handouts.

So:

• un folio = a sheet of paper
• los folios = the sheets/papers

This is common Spanish from Spain. In English, folio is much less common and usually means something more specialized, so you should not translate it too literally in your head.

Los means the, so los folios suggests these are specific papers that are identifiable in the context.

Spanish often uses the definite article where English might say:

• the papers
• some papers
• her papers
• just papers

depending on context.

Here, los folios sounds like the papers are known or relevant to the situation, for example the handouts for the lesson.

Sobre la mesa means on the table, literally on top of the table.

That is a very natural choice here because the papers are physically placed on the table.

You may also hear:

• encima de la mesa = on top of the table

En la mesa can sometimes be used, but it often feels less precise, and in some contexts it can suggest being at the table rather than clearly on top of it.

Also, sobre can mean about in other sentences, but here it clearly means on.

Because in Spanish, singular countable nouns usually need an article when you are talking about a specific thing.

So:

• la mesa = the table

Saying just mesa here would not sound natural.

This is one of the big differences from English: Spanish uses articles more consistently with nouns.

Because la clase is the direct object of empezó here.

In this sentence, empezar is being used as a transitive verb:

• empezar algo = to start something

So:

• empezó la clase = she started the class

You do not use a before a normal direct object like that.

Compare:

• La profesora empezó la clase. = The teacher started the class.
• La clase empezó. = The class started.

In the second sentence, la clase is the subject, not the object.

Yes, by itself it can, because Spanish allows:

• La clase empezó. = The class started.

But in your full sentence, the natural reading is that la profesora remains the subject for both verbs:

• La profesora dejó... y empezó la clase.

So the meaning is:

• The teacher put the papers on the table and started the class.

If the subject changed, Spanish would usually make that clearer.

Because Spanish often leaves out subject pronouns when they are not needed.

The verb form already tells you it is he/she/you (formal), and the subject la profesora has already been stated.

So Spanish naturally says:

• La profesora dejó los folios... y empezó la clase.

not usually:

• La profesora dejó los folios... y ella empezó la clase.

Adding ella would only be used for emphasis, contrast, or clarity.

Because y only changes to e before words that begin with an i sound.

Examples:

• padre e hijo
• siete e incluso ocho

But empezó begins with an e sound, not an i sound, so the normal conjunction stays:

• ...sobre la mesa y empezó la clase

So y is correct here.

The given word order is very natural, but Spanish word order is fairly flexible.

This sentence follows a straightforward pattern:

• subject + verb + object + place + and + verb + object

You could change things for emphasis, for example:

• Sobre la mesa, la profesora dejó los folios y empezó la clase.

That still makes sense, but the original version is the most neutral and natural for ordinary narration.