Learn Hardcore Russian: Многие слушатели активно участвуют в семинаре после лекции. - Many listeners actively participate in the seminar after the lecture.

Because слушатели is the subject of the sentence – those who perform the action. In Russian, the subject typically stands in the nominative case. It’s plural here because многие (“many”) modifies it and indicates more than one listener.

What’s the difference between многие слушатели and много слушателей?

• многие слушатели is a plural phrase where многие (“many” as an adjective-like quantifier) agrees in gender, number, and case with слушатели (nominative plural).
• много слушателей uses много (“a lot of”) as a numeral-quantifier that doesn’t decline; the noun that follows is always in the genitive plural.
Nuance: многие слушатели often stresses “many individual listeners,” while много слушателей simply means “a large number of listeners.”

Why is the verb участвуют in the imperfective aspect?

участвовать is imperfective, focusing on an ongoing, habitual, or repeated action (“take part/participate”). It tells us that the listeners actively take part (perhaps regularly or generally) in the seminar. A perfective alternative like приняли участие would describe a single, completed instance.

Why is активно used here and what part of speech is it?

активно is an adverb describing how the action is performed (“actively”). It’s formed from the adjective активный by replacing the ending –ый with –о. Many Russian adverbs are created this way (e.g., быстрый → быстро, громкий → громко).

What case does в семинаре use and why?

It’s the prepositional case (locative) following the preposition в to indicate “where” someone participates. In addition, the verb участвовать governs в + prepositional whenever you specify the event or activity.

Why is после лекции in the genitive case?

The preposition после always takes the genitive case when indicating time or sequence (“after something”). Here лекция (lecture) becomes лекции (genitive singular) under that rule.

Could you change the word order in this sentence? For example, start with После лекции?

Yes. Russian word order is relatively flexible. You could say:
После лекции многие слушатели активно участвуют в семинаре.
Moving после лекции to the front shifts slight emphasis onto the timing (“It is after the lecture that many listeners…”), but the overall meaning stays the same.

How do you express “the seminar” and “the lecture” without articles in Russian?

Russian has no definite or indefinite articles like the or a/an. Context, word order, intonation, or additional words (e.g., этот семинар “this seminar”) convey whether something is specific. In your sentence, it’s clear from context which seminar and lecture are meant.