Learn Russian: Мы весь день искали кисточку, с которой играла девочка. - We were looking all day for the paintbrush, with which the girl was playing.

The relative pronoun который gets its gender and number from the noun it refers back to (the feminine singular кисточка). However, its case is determined by its job in the new clause. Because the girl was playing with the paintbrush, we need the preposition с (with) followed by the Instrumental case: с которой.

Why is the word order играла девочка instead of девочка играла at the end of the sentence?

In Russian relative clauses starting with a form of который, it is very common and natural to put the verb before the subject, especially when the subject is a full noun rather than a pronoun. The phrase с которой играла девочка flows much better than с которой девочка играла, although both are grammatically correct.

Why is the time phrase весь день in the Accusative case?

To express the duration of an action (how long something happened), Russian uses the Accusative case without any preposition. So, the phrase for 'all day' or 'the whole day' simply becomes весь день.

Why use the imperfective verb искали if they were looking for one specific paintbrush?