Learn Russian: Мы внимательно слушали выступающего.

It is a substantivized participle, which means it is a verb form acting as a noun. It comes from the verb выступать (to speak publicly or perform). The present active participle form is выступающий (the one who is speaking), and here it functions entirely as a standalone noun meaning the speaker.

Why does выступающего take the -его ending if it is the direct object?

The verb слушать takes the accusative case for its direct object. Even though выступающий acts as a noun, it still declines using adjective endings. Because it represents a person (animate masculine), its accusative form matches its genitive form, giving us the -его ending.

Could we use a regular noun like оратор instead of the participle?

Yes, you could say Мы внимательно слушали оратора. However, оратор usually implies a formal, eloquent orator. Выступающий is much more common in everyday professional or academic settings for anyone currently giving a presentation, report, or speech.

Why is the imperfective verb слушали used here instead of the perfective послушали?

The adverb внимательно (carefully or attentively) emphasizes the ongoing process and the quality of the action. The imperfective aspect is a natural fit because the sentence focuses on how the listening unfolded over time, rather than pointing to a completed result.