Breakdown of Тебе нужно собрать свой рюкзак.
ты
you (singular)
нужно
it is necessary
рюкзак
backpack
свой
one's own
Questions & Answers about Тебе нужно собрать свой рюкзак.
Why do we use свой here instead of твой?
The word свой means "one's own" and points back to the person doing the action. Even though this sentence uses an impersonal "to you it is necessary" structure, "you" (тебе) are still the logical person doing the packing. When you do an action to your own object, Russian uses свой.
Why does свой look like it is in the Nominative case if the backpack is the object being packed?
It is actually in the Accusative case! The word рюкзак is the direct object of the verb. Because it is a masculine, inanimate (non-living) noun, its Accusative form is identical to its Nominative form. The pronoun свой simply matches this rule and keeps its base form.
Why does the sentence start with Тебе instead of Ты?
The word нужно (need / it is necessary) requires the person who "needs" something to be in the Dative case. Instead of saying "You need" with the Nominative Ты, Russian grammar literally says "To you it is necessary" using the Dative Тебе.
Why is the perfective verb собрать used here?
The verb собрать is perfective. It is used here because the focus is on the completed result of the action: successfully getting the backpack fully packed as a single, finished event.