"He made a black coffee."
Translation:Gwnaeth e goffi du.
gwnaeth e on its own means 'he made' or 'he did' - it is part of the simple past conjugation of gwneud (making, doing).
mynd, dod, cael and gwneud are the main four irregular verbs in Welsh (apart from bod) and they all have very closely related short-form tense conjugations - see the 'Hints and Tips' notes for one or other of the sections titled 'Past' for a table showing their simple pasts.
There are three basic kinds of initial mutations in Welsh: soft, nasal and aspirate. Look on the web for a summary of these, or hunt around the Duo discussions.
There are 30-40-ish causes of soft mutation, depending on how you count them. This courses covers only some of these - as they come up they are explained in the section notes which you can see on the web-based version of the course.
In this sentence, coffi du is mutated to goffi du because it is the object of the short-form verb gwnaeth.