Aethoch is the past tense, formal 2nd person form of mynd. It's irregular, which might be why you're having trouble! :)
- Dw i'n mynd - I go
- Es i - I went
- Est ti/Aethoch chi - You went
- Aethon ni - We went
- Aethon nhw - They went
- Aethodd e/hi - He/she went
- Ewch - Go (imperative)
It may be worth writing out out some basic verb tables for the very common irregular Welsh verbs: Bod; Mynd; Dod; Cael; Gwneud. Each has common variations, so it is useful to note those as and when you meet them.
You will notice that some patterns begin to emerge among some variations which can be helpful, such as es, des, ces, gwnes i and aethoch, daethoch, caethoch, gwnaethoch chi, etc.
The simple past conjugation of mynd as given in the course is used and understood all over Wales. There are several dialect variations used in various regions as well, too many for a course like this to cover. Really, it is best to avoid those for now unless you know that you will be bumping into them in the wild.
('Northern Welsh' is a fiction - the dialects of Welsh are much more complex and interesting than the mythical north/south divide. Start with the idea that there are 4-5 main dialects, not two.)