"I will make the coffee."
Translation:Gwna i'r coffi.
Gwna i... is the usual form in the general colloquial register of the language. Gwnaf (without the following i) is a more formal form.
People do sometimes write Gwnaf i, but in the interests of simplicity we do not usually use that style in verbs on this course.
There may be a bonus unit on more formal forms, such as those used in writing semi-formal letters, in the Tree2 version of this course that we are working on at the moment. If you are following 'Welsh for Adults' courses, it is covered in Canolradd Uned 9.