In each sentence, the one with "-ly" is proper grammar. "Drive safely." "Drive slowly." "Go quickly." These are all correct. If you want to use proper grammar, you should say these. However... "Drive safe." "Drive slow." "Go quick." These are not technically correct, but sometimes English speakers who are speaking very casually will say them, because the meaning is usually understood. Basically, you can say any of these phrases and people will probably understand. If you want to use proper grammar though, you should use "-ly."