2. I think you be polite to her. She's new in our school and she feels a bit nervous.

2. В данном предложении дается совет, рекомендация, поэтому используем модальный глагол should.

I think you should be polite to her. She's new in our school and she feels a bit nervous.

Ответ: I think you should be polite to her. She's new in our school and she feels a bit nervous.