"Dw i wedi ysgubo yma."

Translation: I have swept here.

[Erated8]

Is Using 'Wedi' In Place Of 'Yn' The Only Way To Form Past-Tense Verbs In Welsh?

[Julianbark6]

"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).

[ibisc]

No, there are several past tenses in Welsh, as in many languages. See the various course notes - https://forum.duolingo.com/comment/17638579