Why "write" is original form in here? Can I use writing or to write?
"Write" here is used because "write" is a present or future tense verb referring to the action of writing. Here, it is used as a future tense verb because he is referring to someone hypothetically writing 1,667 words a day in the future. "Writing" is a "continuous" verb and therefore is used to refer to the present continuous, past continuous, present perfect continuous, etc., so it can not be used here. "To write" is an infinitive and acts as a noun, adjective or adverb, so because "write" needs to be a verb here, "to write" can not be used here. Here are some examples using "write" "writing" and "to write". Ex: She is going to write her essay tomorrow. ("Write" used as a future tense verb) Ex: The students are writing about their favorite food. ("Writing" used as a present continuous verb) Ex: He loves to write. ("Write" used as an infinitive)