Возьми этот инструмент с собой, если собираешься помогать сестре с ремонтом.

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Breakdown of Возьми этот инструмент с собой, если собираешься помогать сестре с ремонтом.

• сестра: the sister
• если: if
• помогать: to help

Questions & Answers about Возьми этот инструмент с собой, если собираешься помогать сестре с ремонтом.

Why is возьми used here, and what tense/mood is it?

Возьми is the imperative (command/request) form of the verb взять (to take). It’s addressing you (singular, informal) and means Take (it)!
It’s from the perfective verb взять, which in imperatives often implies a single completed action (take it once / take it along).

What would be the polite or plural version of возьми?

The polite (or plural “you all”) imperative is возьмите:
Возьмите этот инструмент с собой… = Take this tool with you… (polite / to more than one person).

Could I say бери instead of возьми? What’s the difference?

Yes, but the nuance changes:

• возьми (perfective) = take it (once), make sure you take it
• бери (imperfective imperative of брать) = can sound more like go ahead, take it, or take it (in general / habitually)
  In practical speech, возьми с собой is extremely common for “take with you (don’t forget).”

Why is it этот инструмент (not some other form)? What case is it?