Here are the answers to your questions:
Question 1: When would you use the vous form of you?
Answer: speaking to your friend’s two little brothers (Generally, vous is used in formal situations or when addressing multiple people.)
Question 2: What does Je suis américain mean in English?
Answer: I am American.
Question 3: How would you ask your friend if she is French?
Answer: Es-tu française?
Question 4: What form of être goes with tu?
Answer: es
Question 5: What expression would you use to ask your principal how she or he is doing?
Answer: Comment allez-vous? (This is the formal way to ask someone how they are doing, especially in a formal relationship like with a principal.)