Is Using 'Wedi' In Place Of 'Yn' The Only Way To Form Past-Tense Verbs In Welsh?
"Wedi" is used to form the perfect tense (I have done).
The simple past (I did) is formed by adding particular endings to the verb-stem.
The imperfect (I was doing) uses "Ron i'n" inpace of "Dw i'n" (and similarly for other persons).
No, there are several past tenses in Welsh, as in many languages. See the various course notes - https://forum.duolingo.com/comment/17638579