"You write, my child!"
The "child" is probably someone you know if you're the one saying this. So, it depends of course. I thought "tu" would be correct, and so it was! "Vous" would work, but it's weird to say to perhaps your own child.
j'écris, tu écris, il/elle/on écrit, nous écrivons, vous écrives, ils/elles écrivent