"Os na fydd gen i arian, fydda i ddim yn mynd."

Translation: If I do not have money, I will not go.

[shwmae]

Rather than use ddim as the negator ("not"), os (if) prefers to be followed by na(d).

Fydda i ddim yn mynd (I won't go) Os na fydda i'n mynd (If I won't go / If I'm not going)

Dwyt ti ddim yn hapus (You're not happy) Os nad wyt ti'n hapus (If you're not happy)

Do'n nhw ddim yn gwybod (They didn't know) Os nad o'n nhw'n gwybod (If they didn't know)