If you want to know how to say "I was in..." in Welsh, it would depend on the place you were in.
For example:
* I was in London: Rwy'n bod yn Llundain. (Rwy'n bod is the present tense, but it can also be used to talk about past experiences)
* I was in the park: Rwy'n bod yn y parc.
* I was in Wales: Rwy'n bod yng Nghymru.
Important note: The Welsh language has some complex grammar rules, and depending on the context, the phrase might need slight adjustments.
Do you have a specific location in mind? If you tell me the place, I can give you a more accurate translation.