Formal:
* Arferai fynd: This literally translates to "used to go" and is the most common way to express this in formal situations.
Informal:
* Roeddwn i'n mynd: This translates to "I was going" and is often used in informal conversation to indicate a past habitual action.
* Fe fyddwn i'n mynd: This translates to "I would go" and is another common way to express a past habit in informal speech.
Examples:
* Arferai fynd i'r siop bob dydd Sadwrn. (He used to go to the shop every Saturday.)
* Roeddwn i'n mynd i'r ysgol bob dydd. (I used to go to school every day.)
* Fe fyddwn i'n mynd i'r parc gyda fy ffrindiau. (I would go to the park with my friends.)
Remember that the choice of expression will depend on the context and desired level of formality.