Панельная дискуссия закончилась перед кофе-брейком.

Because the subject is дискуссия, which is a feminine singular noun.

In the past tense, Russian verbs agree with the subject in gender and number:

• masculine: закончился
• feminine: закончилась
• neuter: закончилось
• plural: закончились

So:

• Панельная дискуссия закончилась = The panel discussion ended

Even though панельная дискуссия is a two-word phrase, the main noun is дискуссия, and that noun is feminine, so the verb must be feminine too.

The -сь is the short form of -ся, a very common Russian verb ending.

Here it turns закончить into закончиться.

A useful contrast is:

• закончить = to finish something
• закончиться = to end / to come to an end

So:

• Модератор закончил дискуссию = The moderator finished the discussion
• Дискуссия закончилась = The discussion ended

In your sentence, the discussion itself is the thing that came to an end, so Russian uses закончилась.

Because перед normally requires the instrumental case.

The base form is:

• кофе-брейк = coffee break

After перед, it becomes:

• перед кофе-брейком = before the coffee break

So the ending -ом on брейком shows instrumental singular.

Because кофе-брейк is a hyphenated compound, and in this kind of word, often only the second part takes the case ending.

So:

• nominative: кофе-брейк
• instrumental: кофе-брейком

The first part, кофе, stays the same.

This is especially common with borrowed compounds. Russian treats the second element as the part that gets inflected.

Good question. Кофе is a special noun in Russian because it is usually treated as indeclinable in modern usage when it stands alone.

So by itself, it often stays кофе in all cases.

In кофе-брейк, the inflection happens on брейк, not кофе, so you get:

• перед кофе-брейком

This is normal.

It is in the nominative case, because it is the subject of the sentence.

• панельная agrees with дискуссия
• дискуссия is feminine singular nominative
• so the adjective is also feminine singular nominative: панельная

This whole phrase means panel discussion and functions as the thing that ended.

Панельная comes from панель, and in this context панельная дискуссия means panel discussion.

It is not about a physical panel or wall panel. It refers to a discussion format with several speakers, usually at a conference or event.

So:

• дискуссия = discussion
• панельная дискуссия = panel discussion

This is a fairly standard expression in formal or event-related Russian.

Yes, often you can, but the nuance is a little different.

• перед кофе-брейком = before the coffee break, often meaning right before it
• до кофе-брейка = before the coffee break / up until the coffee break

In many situations they are close in meaning, but:

• перед focuses more on the point just before something
• до often emphasizes the time leading up to that point

So your sentence with перед sounds very natural if the idea is that the discussion ended shortly before the break.

Закончилась is a perfective past tense form.

Russian aspect matters a lot:

• заканчивалась = was ending / used to end
• закончилась = ended, completed

Here the sentence presents the end of the discussion as a completed event, so perfective is the natural choice.

Compare:

• Панельная дискуссия заканчивалась, когда... = The panel discussion was ending when...
• Панельная дискуссия закончилась перед кофе-брейком = The panel discussion ended before the coffee break

Yes. Russian word order is more flexible than English word order because case endings show grammatical roles.

The neutral order here is:

• Панельная дискуссия закончилась перед кофе-брейком.

But you could also hear:

• Перед кофе-брейком закончилась панельная дискуссия.

This might place a little more emphasis on the timing.

In English, changing word order often changes grammar. In Russian, it more often changes focus or style.

A rough pronunciation guide:

• Панельная → pa-NEL-na-ya
• дискуссия → dis-KUS-si-ya
• закончилась → za-KON-chi-las'
• перед → pe-RED
• кофе-брейком → KO-fe-BREY-kom

A few helpful notes:

• The stress in дискуссия is on ку
• The stress in закончилась is on кон
• The stress in кофе-брейком is on брей

So the sentence rhythm is roughly:

pa-NEL-na-ya dis-KUS-si-ya za-KON-chi-las' pe-RED KO-fe-BREY-kom

They are related, but not always identical.

• дискуссия usually means a discussion involving exchange of views, argument, or debate
• обсуждение is broader and often means discussion in the sense of considering a topic

At a conference, панельная дискуссия is the standard way to say panel discussion.
You would not normally replace it with панельное обсуждение in the same way.

A useful learner breakdown is:

• закончи- = verb stem
• -л- = past tense marker
• -а = feminine singular
• -сь = reflexive ending

So закончилась tells you several things at once:

• it is past
• it is feminine singular
• it is the reflexive/intransitive form

That is why one Russian word can correspond to English ended in this sentence.