Yn ôl plus a length of time means ago and points at a finished moment: ychydig flynyddoedd yn ôl. Ers plus the same phrase means for that long and describes something still true: dw i yma ers tair blynedd.
Examples
- Ychydig flynyddoedd yn ôl ro'n i'n byw yn Abertawe.
- A few years ago I was living in Swansea.
- Dw i'n byw yma ers tair blynedd.
- I have been living here for three years.
- Pan o'n i'n ifanc, es i i'r traeth bob haf.
- When I was young, I went to the beach every summer.
The mistake to avoid
Yn ôl is finished; ers describes something still going on.
Where this fits in
Y gorffennol a'r penwythnos diwethaf, Welsh A2: Talk about the past: yesterday, last weekend, and travel memories.