Коль скоро пленарное заседание закончилось, начинается панельная дискуссия.

Here коль скоро is a conjunction meaning something like:

• since
• now that
• seeing that

So the sentence means roughly: the plenary session has ended, so the panel discussion is beginning.

A useful note: коль скоро is quite formal or bookish. It is not the most everyday way to say this. In more neutral Russian, you might more often see поскольку, раз, or just no special conjunction at all, depending on context.

Not especially in ordinary conversation.

It is understood, but it sounds:

• formal
• somewhat literary
• slightly old-fashioned or rhetorical in some contexts

In speech, many Russians would be more likely to say:

• Поскольку пленарное заседание закончилось, начинается панельная дискуссия.
• Раз пленарное заседание закончилось, начинается панельная дискуссия.
• Пленарное заседание закончилось, начинается панельная дискуссия.

So yes, it is correct, but stylistically elevated.

Because Коль скоро пленарное заседание закончилось is a subordinate clause, and it comes before the main clause:

• subordinate clause: Коль скоро пленарное заседание закончилось
• main clause: начинается панельная дискуссия

In Russian, subordinate clauses are normally separated by commas.

This is a very natural thing in Russian.

• закончилось = ended / has ended
• начинается = is beginning / begins

Russian often uses the present tense for:

• events happening right now
• scheduled or program-like announcements
• narration of what comes next

So the idea is: the plenary session is over, and now the panel discussion is starting.

In English, depending on context, we might say:

• Now that the plenary session has ended, the panel discussion is beginning.
• Since the plenary session is over, the panel discussion is starting.
• The plenary session has ended; the panel discussion now begins.

Yes. Начнётся is also possible, but it changes the feel slightly.

• начинается = is beginning / now begins
  This sounds immediate, like an announcement or live sequence of events.
• начнётся = will begin
  This sounds a bit more future-oriented.

So:

• ...начинается панельная дискуссия = the panel discussion is starting now
• ...начнётся панельная дискуссия = the panel discussion will start

The version in your sentence feels like a moderator or program announcement.

Because adjectives in Russian agree with the noun they describe in:

• gender
• number
• case

Here both noun phrases are in the nominative singular, but the nouns have different genders:

• заседание is neuter
  so the adjective is пленарное
• дискуссия is feminine
  so the adjective is панельная

That is why the endings are different:

• neuter nominative singular adjective: -ое
• feminine nominative singular adjective: -ая

Both are in the nominative case.

Why?

Because each phrase is the subject of its clause:

• пленарное заседание закончилось
  the plenary session ended
• панельная дискуссия начинается
  the panel discussion is beginning

Subjects are normally in the nominative case in Russian.

Because it comes from the verb закончиться.

This verb has two important features:

1. It is past tense, neuter singular
2. It is reflexive, marked by -ся / -сь

Breakdown:

• закончил- = past tense stem
• -о = neuter singular past ending
• -сь = reflexive marker

It agrees with заседание, which is neuter singular.

So:

• заседание закончилось = the session ended

The reflexive form is very common with events that begin or end:

• урок начался = the lesson began
• встреча закончилась = the meeting ended
• заседание закончилось = the session ended

Because the sentence refers to a completed event.

• закончилось is perfective: the plenary session reached its end
• an imperfective form would describe process, repetition, or background, not a single completed result

Russian uses the perfective here because the ending of the plenary session is treated as a finished fact that allows the next event to begin.

That is exactly the kind of meaning perfective verbs often express.

Yes, the word order is normal.

Russian word order is more flexible than English, and putting the subordinate clause first is very common, especially in formal writing:

• Коль скоро пленарное заседание закончилось, начинается панельная дискуссия.

This order gives the reason or condition first, then the main event.

You could also reorder it:

• Начинается панельная дискуссия, коль скоро пленарное заседание закончилось.

But that sounds less natural here. The original order is much better.

It is understandable and grammatically correct, but stylistically it sounds quite formal.

What sounds natural about it:

• the grammar is fine
• the conference vocabulary is normal
• пленарное заседание and панельная дискуссия are both standard expressions

What stands out:

• коль скоро sounds elevated or bookish

So it does not sound wrong, but it does sound more official, written, or rhetorical than everyday spoken Russian.

Yes, both are standard, especially in academic, political, and conference settings.

• пленарное заседание = plenary session
• панельная дискуссия = panel discussion

These are common in:

• conferences
• forums
• academic events
• official programs

So the vocabulary itself is completely normal for that context.

Some more neutral or everyday alternatives are:

• Поскольку пленарное заседание закончилось, начинается панельная дискуссия.
• Раз пленарное заседание закончилось, начинается панельная дискуссия.
• Пленарное заседание закончилось, начинается панельная дискуссия.
• После пленарного заседания начинается панельная дискуссия.

These differ slightly in tone:

• поскольку = neutral-formal since
• раз = conversational since / if
• no conjunction = direct, announcement-like
• после = focuses on sequence rather than reason

A helpful stress pattern is:

Коль скóро пленáрное заседáние закончилóсь, начинáется панéльная дискýссия.

Main stressed syllables:

• скóро
• пленáрное
• заседáние
• закончилóсь
• начинáется
• панéльная
• дискýссия

For a learner, the most important ones to remember are probably:

• заседáние
• закончилóсь
• начинáется
• дискýссия