"Ar ôl i Dewi Lingo fynd, es i adre."

Translation: After Dewi Lingo went, I went home.

[MatthewSke1]

Why is the "f" on the end of "adref" dropped?

[ibisc]

Probably because it is not normally pronounced, or is pronounced only very lightly, in the spoken language. I tend to leave it in when writing, and it is never wrong to do so even if it is optional.