Learn Hardcore Russian: Пожалуйста, попроси учителя повторить правило. - Please ask the teacher to repeat the rule.

Попроси is the 2nd person singular imperative of perfective попросить (used with ты). It asks for a single, complete action: “ask (once).” For polite/plural вы, use попросите. The imperfective imperative проси would suggest a process or repeated action, not a one‑off request.

Why is it учителя and not учитель—or у учителя?

With the pattern попросить кого сделать что (“ask someone to do something”), the person is a direct object in the accusative. For animate masculine nouns, the accusative singular = genitive singular, hence учителя. The pattern попросить у кого чего (“ask someone for something”) uses у + genitive and no infinitive: Попроси у учителя мел (“ask the teacher for chalk”).

Why is повторить used, not повторять?

Повторить is perfective and requests one completed repetition. Повторять is imperfective and would mean to keep repeating or do it habitually: Попроси учителя повторять правило suggests a regular action, which isn’t intended here.

What case is правило, and would it change in the plural?