Learn Russian: Кто-нибудь знает, что говорит лектор, держащий микрофон? - Does anyone know what the lecturer holding the microphone is saying?

To form a present active participle, you look at the "they" form of the verb in the present tense. For держать (to hold), the "they" form is держат. You drop the final -т and add the ending -щий. Verbs ending in -ат or -ят in their "they" form always take -ащ- or -ящ- endings to form this participle.

Why is держащий in the nominative case here?

Participles decline just like adjectives, meaning they must match the gender, number, and case of the noun they modify. Here, it describes the лектор (lecturer). Because the lecturer is the subject of the clause doing the speaking (лектор говорит), лектор is in the nominative case. Therefore, the participle takes the matching masculine singular nominative form держащий.

Why is there a comma right before держащий?

In Russian, when a participial phrase (the participle plus any words attached to it, like держащий микрофон) comes directly after the noun it describes (лектор), it must be set off by a comma. If you changed the word order to put the phrase before the noun (что говорит держащий микрофон лектор), you would drop the comma.

Could we use кто-то instead of кто-нибудь in this sentence?