"He definitely writes books."
Translation:Scríobhann sé leabhair go cinnte.
Clefting the sentence renders the go extraneous, since there’s an implied is at the beginning of a clefted sentence — Cinnte a scríobhann sé leabhair is really (Is) cinnte a scríobhann sé leabhair. (Note that when clefting a sentence, there should be an a before the verb, because the non-clefted part becomes a direct relative clause.)
Some people have hinted at this question, but I'll ask it clearly:
He (as opposed to she) definitely writes books.
He definitely writes (as opposed to reads) books.
He definitely writes books (as opposed to magazines).
How would the Irish translation differ for the above 3 variations?
Duolingo isn't designed to test your knowledge of Irish that you learned elsewhere. If you report additional synonyms that can be used to say the same thing as an exercise, the contributors might add them as alternative answers.
(You have to use the "Report" button when you encounter the exercise - there is no guarantee that the course contributors who have access to modify the exercises will ever see your comments here).
It really is helpful for other learners to introduce these alternative expressions in the discussions (even if they are accepted as alternative answers, they won't usually be presented to other learners in the exercises). Sometimes someone will be able to point out that a particular expression is mainly only used in one particular dialect, or that that it may have some other conflicting meaning that you mightn't be aware of.
And sometimes it's just another way of saying the same thing.
Yes, I understand, and I would do so, and will do so if I come across it again, but unfortunately because of the structure of Duolingo (which is presumably to do with its interface and the realities of programming), you can't reverse back into the lesson and add a report after passing on from a particular phrase. Go deimhin is a nice one for people coming to Irish from English or French, because it is so near definitely and définitivement.