"Ar ôl i Dewi Lingo fynd."
Translation:After Dewi Lingo went.
This sort pattern with i is a common one in Welsh. In this course you have only met it with rhaid, ar ôl, and cyn, but it is used in many other phrases as well.
On its own it is the pronoun I/me, and also a preposition to/for. In this pattern it is the preposition, but it is not directly translated into English. It just links the ar ôl to whomever the verb refers to. Another example of Welsh and English using very different patterns to express the same meaning.