Paid and Peidiwch are followed by â or ag.

ag is used in front of a vowel, e.g.

Peidiwch ag yfed a gyrru.

Paid ag anghofio dy wreiddiau pan fyddi di yn y coleg.

â is used in front of a consonant and is followed by an aspirate mutation, e.g.

Peidiwch â phoeni am y gwaith.

Paid â chredu popeth sy'n cael ei ddweud am y cwrs.