As explained in some of the section notes, verb-nouns such as dewis, particularly when they appear on their own, are probably best though of as meaning 'choosing' or 'to choose'. That is the form that we use on this course, since that best reflects the function of the word. In dictionaries, yes, 'choose' = dewis, but then a dictionary will indicate that the word is a verb-noun rather than a noun or a command form of the verb - Duo cannot do that.
i ddewis would always occur as part of a longer phrase or sentence:
- Dw i'n mynd i ddewis - 'I am going to choose', where the 'i belongs with mynd rather than with dewis.
- Dw i'n mynd i'r dref i ddewis crys newydd - 'I am going to town to choose a new shirt' - ...mynd i... - ...going to (a place); and ...i ddewis... - '...(in order) to choose...', where i introduces the purpose of the going into town.