Breakdown of Зачем ты тащишь этот мешок на спине? Возьми тележку!
ты
you (singular)
этот
this [f: эта, n: это]
на
on, at / by (transport) / to (a place) / to (in answer to)
взять
to take / to adopt
Questions & Answers about Зачем ты тащишь этот мешок на спине? Возьми тележку!
Why use the unidirectional verb тащишь here instead of the multidirectional таскаешь?
The speaker is reacting to an action happening right before their eyes, moving in one specific direction. The unidirectional тащить focuses on this single, ongoing trip. If you used таскаешь, it would imply the person hauls sacks regularly as a habit, or is wandering around aimlessly with it.
Why is the imperative verb Возьми perfective here, while тащишь is imperfective?
Тащишь is imperfective because it describes a continuous action happening right now. However, Возьми (the perfective form of брать) is used because the speaker is urging the person to do a single, completed action right away: get the cart.
Why do we use на спине (prepositional case) instead of the instrumental case?
In Russian, carrying something on your back or on your shoulders is treated as a location, so it takes на plus the prepositional case. The instrumental case would sound like you are using your back as an active tool to manipulate the sack, rather than just resting the heavy load on it.
Could I say Почему instead of Зачем at the beginning of the sentence?
Yes, but Зачем is a better fit here. Почему broadly asks for the cause, whereas Зачем specifically asks for the purpose or goal. The speaker is questioning the practical logic of struggling with the bag when a cart is available.