Negative verbs starting with t, c and p take an aspirate mutation, e.g.

Chlywais i ddim am y ddamwain.

Phrynais i ddim byd ddoe.

Ddim / mo / mo'r

When the object is indefinite (e.g. a film, a book etc.), use ddim, e.g.
Welais i ddim ffilm neithiwr. (I didn't see a film last night.)

When the object is definite (e.g. the film, the book), use mo'r, e.g.
Welais i mo'r ffilm neithiwr. (I didn't see the film last night.)

When the object is named (e.g. Star Wars, War and Peace), use mo, e.g.
Welais i mo Star Wars neithiwr. (I didn't see Star Wars last night.)

When the definite object is a pronoun (he, she, them …), i.e. a word used to replace a noun that has already been mentioned, the preposition o conjugates as follows in a negative context:

mohono i	Welaist ti mohono i	You didn't see me
mohonot ti	Welais i mohonot ti	I didn't see you
mohono fe/fo	Welais i mohono fe/fo	I didn't see him
mohoni hi	Welodd e mohoni hi	He didn't see her
mohonon ni	Welsoch chi mohonon ni	You didn't see us
mohonoch chi	Welodd hi mohonoch chi	She didn't see you
mohonyn nhw	Welais i mohonyn nhw	I didn't see them