It's used in its mutated form with verbs where it corresponds to the English "not" e.g Dw i ddim yn hoffi coffi. With a noun it's just "dim" e.g. dim plant (no kids), dim amser (no time), dim diolch (no thanks).
i accidentally put not instead of no and it said i was wrong, i am quite fluent in wlesh anyways but i am just getting to know every single corners of this great language