Кофе-брейк будет после пленарного заседания.

That’s right: in the present tense, Russian usually omits to be.

• Он студент. = He is a student.
• Кофе-брейк после заседания. would sound incomplete in this context unless used like a note or headline.

But in the future tense, Russian does use forms of быть:

• будет = will be for he/it and also in impersonal statements like this one

So:

• Кофе-брейк будет после пленарного заседания.
  = The coffee break will be after the plenary session.

Here будет is necessary because the sentence is about a future event.

It is genitive singular.

That happens because the preposition после always takes the genitive case.

• после чего? = after what?

So:

• пленарное заседание = nominative singular
• после пленарного заседания = genitive singular after после

Both words change because the adjective must agree with the noun:

• пленарное → пленарного
• заседание → заседания

Because заседание is a neuter noun, and here it must be in the genitive singular after после.

Pattern:

• nominative singular: -ие / -ание nouns like заседание
• genitive singular: usually -ия / -ания

So:

• заседание → заседания

This is a very common change for neuter nouns ending in -ие or -ание.

Because adjectives in Russian must agree with the noun in:

• gender
• number
• case

The noun заседание is:

• neuter
• singular
• genitive

So the adjective пленарный has to match that:

• nominative neuter singular: пленарное
• genitive singular: пленарного

Even though -ого often looks masculine/neuter in the genitive singular, here it is correct because genitive singular neuter uses the same ending as masculine.

После means after.

It is a preposition that normally takes the genitive case:

• после обеда = after lunch
• после встречи = after the meeting
• после лекции = after the lecture

So in your sentence:

• после пленарного заседания = after the plenary session

For a learner, the key thing to remember is simple:

• после + genitive

Yes, it is used in modern Russian, especially in:

• business contexts
• conferences
• formal events
• schedules and announcements

It is a borrowing from English coffee break.

You may also hear a more native-sounding expression:

• перерыв на кофе

Both can mean essentially the same thing, but кофе-брейк sounds more like conference or corporate language.

Because it is a borrowed compound word, and Russian often writes these newer borrowed compounds with a hyphen.

Examples of similar structure include items like:

• бизнес-центр
• интернет-магазин
• пресс-конференция

So кофе-брейк is just the standard written form of that borrowed expression.

In practice, many speakers treat кофе-брейк as an indeclinable borrowed expression, especially in modern business speech.

So you may often see it stay the same:

• кофе-брейк
• после кофе-брейк would sound awkward to many speakers, though, because people often avoid forcing case endings onto this type of borrowing.

Instead, Russian often prefers to rephrase:

• после кофе-брейка may be heard
• but many speakers would rather say после перерыва на кофе

For your sentence, none of that matters directly, because кофе-брейк is the subject and appears in the nominative.

Yes. Russian word order is flexible, and changing it changes the emphasis more than the core meaning.

Your sentence:

• Кофе-брейк будет после пленарного заседания.

This is perfectly natural and focuses first on the coffee break.

You could also say:

• После пленарного заседания будет кофе-брейк.

This version starts with after the plenary session, so it feels a bit more like answering the question What happens after the plenary session?

Both are correct. The difference is mainly emphasis and information flow.

Yes, it sounds natural, especially in an event or conference setting.

It has a fairly neutral-to-formal tone because of words like:

• кофе-брейк
• пленарное заседание

This is exactly the kind of sentence you might hear:

• at a conference
• in a schedule announcement
• from an event organizer
• in a program

A slightly more traditional or less Anglicized version could be:

• Перерыв на кофе будет после пленарного заседания.

That may sound a bit more native-Russian to some speakers, but your original sentence is absolutely normal.

Because Russian has no articles.

English uses:

• a
• the

Russian does not. So Russian relies on:

• context
• word order
• intonation
• the situation

That means Кофе-брейк будет после пленарного заседания can mean:

• The coffee break will be after the plenary session
• or, depending on context, A coffee break will be after the plenary session

In most real situations, context makes it clear that a specific scheduled coffee break and a specific plenary session are meant.

The usual stress is:

• ко́фе-брейк
• бу́дет
• по́сле
• плена́рного
• заседа́ния

So the full sentence is pronounced approximately like this:

• КО́фе-брейк BÚdet PÓsle plenárnava zasedániya

A useful thing to notice is that the adjective ending -ого in standard pronunciation is often pronounced more like -ова:

• пленарного sounds like плена́рнава

That is normal in standard Russian pronunciation.

Yes, that is the literal structure, and it can help you see the grammar:

• Кофе-брейк = coffee break
• будет = will be
• после = after
• пленарного заседания = of the plenary session / after the plenary session

But in natural English, you would normally say:

• The coffee break will be after the plenary session.

So the literal translation is useful for learning the structure, but not as the best final English version.