The preposition i is used with many other words as well as rhaid. We need to conjugate i after cyn (before) and ar ôl (after). Remember that the verb will mutate:
Rhaid i mi fynd.
I have to go.
Cyn i mi fynd.
Before I go.
Cyn
Ar ôl
I
Cyn i mi fynd.
Ar ôl i mi fynd.
You
Cyn i ti fynd.
Ar ôl i ti fynd.
He
Cyn iddo fo fynd.
Ar ôl iddo fo fynd.
She
Cyn iddi hi fynd.
Ar ôl iddi hi fynd.
We
Cyn i ni fynd.
Ar ôl i ni fynd.
You (plural/formal)
Cyn i chi fynd.
Ar ôl i chi fynd.
They
Cyn iddyn nhw fynd.
Ar ôl iddyn nhw fynd.
Be' wnest ti ar ôl i ti gael swper?
What did you do after you had supper?
Be' wnaeth y plant cyn iddyn nhw fynd i'r ysgol?
What did the children do before they went to school?