Breakdown of Она должна простить брата за такие шутки.
брат
brother
она
she / it (for a feminine noun)
шутка
joke
должен
must, have to (должен, должна, должны)
Questions & Answers about Она должна простить брата за такие шутки.
Why do we use за here, and what case does it take?
The preposition за is used here to show the reason for the forgiveness, just like it is used to express gratitude or the price of an exchange. It requires the Accusative case, which is why we see такие шутки. Since шутки is an inanimate plural noun, its Accusative form looks exactly like its Nominative form.
Why does брат change to брата in this sentence?
The verb простить takes a direct object, meaning the person being forgiven must be in the Accusative case. Because брат is an animate masculine noun, a living being, its Accusative ending is -а. This makes it look identical to the Genitive case.
Why use the perfective verb простить instead of the imperfective прощать?
We use the perfective простить because the sentence refers to a specific, one-time result: she needs to successfully forgive him for these specific jokes right now. If the sentence meant she must generally or always forgive him, it would use the imperfective прощать.