write.down; write.on; write.in

write……down：
And let me write them down.
让我把它们记下来.
And write down your room number.
并写上房号.
write……on：
I will write on for you.
我开给您.
Write the address on the envelope.
在信封上写上地址.
write……in：
Can you write in English?
你能用英文写文章吗?
Write your initials in capitals.
把你名字的第一个字母写成大写.
这样应该很能告诉你他们的区别了吧?