I put "Toho medvěda pro ni potřebuju," and it was marked incorrect. The only difference between this and the correct answer, "Toho medvěda potřebuju pro ni," is the word order, and in almost every exercise in this lesson, potřebuj- or nepotřebuj- has been the last word in the sentence. Why is this incorrect?

I tried "Já toho medvěd potřebuju pro ji." I know (now) it should be medvěda but is "Já" correct?

What is the difference between 'ji' and 'ni'