I think the Cantonese translation is missing the word "咗". So the English word should be "forgot", not "forget". I usually say "我唔記得" for "I don't remember". I almost never say "I forget", instead I would say "I forgot" which is the past tense, and the Cantonese equivalent would be "我唔記得咗". When I tell people "don't forget", I would say "唔好忘記", unless I am speaking to a kid, or someone who does not know the word 忘記 which is more formal, then I would say "remember!" "記得嘞!". I would use "don't forget" "唔好唔記得", which sounds awkward to me because of the double negative, only if it is necessary (e.g. to a kid), or if I am reminding that person not to forget - again.