"I got up at seven o'clock."
Translation:Codais i am saith o'r gloch.
No, no soft mutation needed for the past tense. What you have probably seen are things like Wnes i fynd... or Ges i fy ngeni... and so on. These are quite often mutated in the wild because of particles which are not always used now, but which do cause soft mutation if they are there - and optionally even if they are not...! So:
Mi godais i am saith... - mi indicating that what follows is an affirmative verb, more often used in parts of north and mid-Wales still. No need to use it unless those around you do so, but useful to be aware of it.
Fe godais i am saith... - fe doing exactly the same thing, more often used in parts of south-west and south Wales, less used nowadays. No need to use it unless those around you do so, but useful to be aware of it.
Godais i am... - mutated by some people who believe in the ghost of mi/fe, and commonly seen with Ges i..., and Wnes i.... Use Ges i... and Wnes i if you like, probably don't bother with it with other verbs, but be aware it can occur.
Godais i am...? - mutated because it is a question (following another ghostly particle, but a question marker one this time (a)). Don't use this particular a these days except in formal writing, but do use the mutation for question-shaped verbs
Chodais i ddim am saith... - a negative verb, mutated because of a ghostly negative particle (ni). Quite often in the wild you with come across Godais i ddim..., perhaps because of ghostly dialect variations in the mutations after ni. Don't use ni these days except in formal writing. Do use whatever mutation you have been taught for negative verbs.