Why is "he writes his fifth book" incorrect? Is that not also a valid translation of the sentence?
I would use "he writes" for something done regularly, such as, "he writes her a letter each week." In this case, since one's fifth book can only be written once, the right thing to say is "he is writing."
thanks for your thoughtful response. i like to use the absurdity of He often writes his fifth book. as a screening tool for the applicability of the present simple. perhaps it could also help others feel the difference without having to think too much.
It's sort of correct (and given that this is a course in Czech and not English, I'd say it should be accepted), but only if it's a very short book. "He writes" is the simple aspect, which is used for more of a one-and-done kind of action. "He is writing", on the other hand, is the progressive or continuous aspect, which is used for an ongoing process. Given that writing a book typically takes time, it's much more natural to use the progressive aspect.