"Mi wnaeth hi fwyta swper."
Translation:She ate supper.
The 'mi' and 'hi' confuse me. Why are there two pronouns and how do I know which one the sentence is about?
The mi at the beginning is not (nowadays) a pronoun -- it's just a pre-verbal particle or marker to show that this sentence is a statement (and not a question or command).
So the only pronoun here is hi.
Remember that Welsh sentences nearly always begin with a verb, so if the subject is a pronoun, it will almost always come after the verb.
For example, dw i, fyddi di, gwnaeth hi etc.