"After I got up, I had porridge."
Translation:Ar ôl i mi godi, ges i uwd.
Think of it, perhaps, as "after getting up" -- no change for past, present, or future.
"After getting up, I had porridge" -- the "after getting up" must be past.
"After getting up, I will have porridge" -- the "after getting up" must be future.
The "getting up" doesn't change but uses the gerund (noun form) of the verb. And in Welsh, this construction also uses the verbal noun.