Learn Turkish: Bizim almamız gereken pudra bitti. - The powder we must buy is finished.

This is the core structure for relative clauses of necessity taught in this lesson. You take the verb stem al- (buy), add the verbal noun ending -ma, and then the possessive ending for "our" (-mız). You then follow it with gereken (which literally means "that is necessary"). So, almamız gereken pudra literally translates to "the powder that our buying is necessary", which flows much better in English as "the powder that we must buy".

Why does the sentence use Bizim instead of Biz?

Because this type of relative clause in Turkish relies on a possessive noun phrase structure. Since we turned the verb into a noun (almamız meaning "our buying"), the person performing that action has to take the genitive (possessive) case. So "we" (biz) becomes "our" (bizim).

Could I drop the word Bizim entirely from this sentence?

Yes, you can. Because the possessive suffix -mız on almamız already makes it perfectly clear that "we" are the ones who need to do the buying, the word bizim is technically optional. Including it just adds a little extra emphasis, like saying "the powder that we must buy".

Why is bitti in the past tense if the translation says "is finished"?