根据提示填空（每空一词）
你已经决定英语课写哪本书了吗？
Have you decided yet which book ____ ____ about for English class?

本句可用“特殊疑问词+to do不定式”结构来表达。所以“写哪本书”可以翻译成“which book to write about”。“which book to write about for English class”在句中作decide的宾语。所以答案为to write。

略