En cuanto termina la mesa redonda, empieza la presentación editorial.

En cuanto is a time expression meaning as soon as.

It introduces a clause that marks the moment one thing happens, and then the next thing happens immediately after:

• En cuanto termina la mesa redonda, empieza...
• As soon as the panel discussion ends, ... starts

A common learner mistake is trying to interpret en cuanto word by word. In this expression, you should learn it as a fixed chunk.

This is a very good question, because learners are often taught that after time expressions like cuando, en cuanto, tan pronto como, etc., Spanish often uses the present subjunctive for future events:

• En cuanto termine la mesa redonda, empezará la presentación editorial.

That version is also very natural.

The sentence you have uses termina instead, which can sound like:

• a scheduled program
• a habitual sequence
• a timetable-style present
• a vivid, agenda-like description of what happens next

So this sentence feels a bit like describing the running order of an event. In other words, it presents the sequence as fixed and programmatic.

Spanish often uses the present tense for scheduled or arranged future events, especially in programs, timetables, and event descriptions.

For example:

• La película empieza a las ocho.
• Mañana empieza el curso.

So empieza here does not have to mean a literal present happening right now. It can mean starts next in the schedule.

You could also say:

• ... empezará la presentación editorial

That sounds a bit more explicitly future, while empieza sounds more immediate and agenda-like.

Mesa redonda literally means round table, but in modern Spanish it very often means a panel discussion or round-table discussion.

So in event or conference language, la mesa redonda is not usually about furniture. It is a discussion format with several speakers.

Here editorial is an adjective, and it is related to publishing.

So presentación editorial is something like:

• a publishing presentation
• a publisher’s presentation
• an editorial presentation

Exactly how you translate it depends on context. In Spanish, editorial can refer to the publishing world, not just to a newspaper editorial article.

So here editorial probably does not mean opinion piece. It most likely refers to a presentation connected with a publisher, a book project, or the publishing industry.

The subject is la presentación editorial.

So the structure is:

• empieza = begins / starts
• la presentación editorial = the editorial/publishing presentation

Spanish often puts the verb before the subject, especially when talking about events beginning, ending, arriving, happening, etc.

So:

• Empieza la presentación editorial
• La presentación editorial empieza

Both are possible. The first one is very natural.

Also, in the first clause, the subject of termina is la mesa redonda.

Because la presentación editorial is the subject, not an indirect object or a verb complement with a.

Compare:

• Empieza la presentación editorial.
  = The presentation starts.

But you do use a in other cases with empezar, for example:

• Empieza a las cinco.
  = It starts at five.
• Empieza a hablar.
  = He/She starts to speak.

So in your sentence, there is no a because la presentación editorial is simply the thing that begins.

Spanish uses the definite article more often than English does.

Here la mesa redonda and la presentación editorial refer to specific, identifiable parts of an event program. That is why the article sounds natural.

In English, you might sometimes omit the article in headlines or schedules, but Spanish normally keeps it:

• la mesa redonda
• la presentación editorial

It helps signal that these are known, specific event items.

Yes, you could say:

• Cuando termina la mesa redonda, empieza la presentación editorial.

That is grammatical, but en cuanto gives a stronger sense of immediately after or as soon as.

So the difference is roughly:

• cuando = when
• en cuanto = as soon as

If the idea is that one event starts right after the other, en cuanto is the more precise choice.

Because the sentence starts with a subordinate time clause:

• En cuanto termina la mesa redonda, ...

In Spanish, when this kind of subordinate clause comes first, a comma is normally used before the main clause.

If you reverse the order, the comma is often dropped:

• Empieza la presentación editorial en cuanto termina la mesa redonda.

So the comma is about sentence structure, not about a pause in speech only.

Yes. Empieza and comienza both mean starts/begins.

• Empieza la presentación editorial
• Comienza la presentación editorial

Both are correct. In many contexts:

• empezar sounds a bit more everyday and common
• comenzar can sound slightly more formal or written

In a conference or event program, either one would work well.