To teach Japanese

Today, I taught Japanese to my friend who was from China. It was quite difficult. I had to explain about Japanese gramer in English... From the begining, I explained about Hira-gana and Kata-kana. He asked me many questions, for example, "Why do you have two letters? What is the difference?""Why do you have so many exoression? 僕、俺、私.. What is the difference?""How do you distinguish which letter is better for each word? What is the difference?" I really confused... I tried to explain as much as I could. Japanese is a so complicated language.