Questions & Answers about Где она? Я её не вижу.
Why is it её instead of она in the second sentence?
её is the Accusative case form of она (she). We use the Accusative case for the direct object of a sentence, which is the person or thing receiving the action. Since she is the person being seen, she becomes её (her).
Wait, doesn't её also mean 'her' as in 'her car'?
Yes! The word её does double duty. It is used as the Accusative pronoun meaning 'her' (the object of an action, as seen here) and also as a possessive word meaning 'her' or 'hers' (like in её дом - her house). Context tells you which one it is.
Why is её placed before не вижу? Can I say Я не вижу её?
Yes, you can absolutely say Я не вижу её. Russian word order is flexible, but pulling the object pronoun to the front of the verb is very common and often sounds more natural in short, simple sentences like this one.
Why does the verb вижу end in -у? Doesn't the я (I) form usually end in -ю?