Пленарное заседание началось вовремя.

Because пленарное is an adjective, and it has to agree with заседание in:

• gender: neuter
• number: singular
• case: nominative

The dictionary form is пленарный.
Its neuter singular nominative form is пленарное.

So:

• пленарный = masculine
• пленарная = feminine
• пленарное = neuter
• пленарные = plural

Заседание is a neuter noun. A very common clue is the ending -ие: many nouns ending in -ие are neuter.

So here:

• заседание = neuter singular noun
• therefore the adjective becomes пленарное
• and the past-tense verb becomes началось

This is one of the important patterns in Russian: singular past-tense verbs agree with the gender of the subject.

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

You can ask yourself: What began on time?
Answer: Пленарное заседание.

That makes it the subject, so nominative is the expected case.

Because the subject заседание is neuter singular.

In the past tense, Russian verbs change for:

• gender in the singular
• number in the plural

So from начаться you get:

• начался = masculine
• началась = feminine
• началось = neuter
• начались = plural

Since заседание is neuter, the correct form is началось.

It comes from the verb начаться, which is the reflexive/intransitive partner of начать.

A useful contrast is:

• начать что-то = to start something
• начаться = to begin, to start

So:

• Организаторы начали заседание = The organizers started the session.
• Заседание началось = The session began.

In this sentence, the session is not the thing being actively started by a direct object construction. Instead, it is the thing that began, so Russian uses начаться.

Also, -сь is just a phonetic variant of -ся used after a vowel.

This is about aspect.

• началось is perfective
• начиналось is imperfective

Here, началось is natural because the sentence refers to a completed event at a definite moment: the session started, and it did so on time.

Using начиналось would change the meaning. It might suggest:

• a repeated/habitual idea
• a process in progress
• background description rather than a single completed starting point

So in this sentence, началось is the normal choice.

Because вовремя is an adverb meaning on time or punctually.

This is very important because Russian also has во время as two words, and that means something different:

• вовремя = on time
• во время
  • genitive noun = during

Compare:

• Заседание началось вовремя. = The session began on time.
• Во время заседания никто не выходил. = During the session, nobody left.

This is one of the most common spelling distinctions learners need to remember.

This is mainly for pronunciation ease and reflects an older prepositional form.

Russian often uses во instead of в before certain consonant clusters to make the word easier to say. In modern Russian, you mostly just learn forms like this as fixed expressions.

So the important practical point is:

• memorize вовремя as a single adverb meaning on time
• do not confuse it with во время meaning during

No, Russian word order is fairly flexible, but the given order is very natural and neutral:

Пленарное заседание началось вовремя.

This is a straightforward subject + verb + adverb structure.

You could change the order for emphasis, for example:

• Вовремя началось пленарное заседание.
• Пленарное заседание вовремя началось.

These are possible, but they sound more marked or stylistically different. For a neutral statement, the original order is best.

It is an adverb.

It modifies the verb началось and tells you how or under what timing condition the action happened:

• началось = began
• вовремя = on time

Unlike nouns or adjectives, adverbs do not change for case, gender, or number.

The stress is:

• плена́рное
• заседа́ние
• начало́сь
• вовре́мя

A rough pronunciation guide:

• плена́рное ≈ pli-NAR-nə-ye
• заседа́ние ≈ zə-si-DA-ni-ye
• начало́сь ≈ nu-chi-loS'
• вовре́мя ≈ va-VRYE-mya

If you are practicing aloud, pay special attention to the final stress in начало́сь and the middle stress in вовре́мя.

Yes. Заседание often means something like:

• session
• meeting
• sitting in formal contexts

In the phrase пленарное заседание, the most natural English equivalent is usually plenary session.

So while заседание by itself can sometimes be translated in different ways depending on context, in this phrase session is usually the best fit.

Because Russian does not have articles like the and a/an.

So Пленарное заседание can mean:

• the plenary session
• a plenary session

Which one is meant depends on context. In real usage, the surrounding situation usually makes it clear whether the speaker means a specific session or just a plenary session in general.