How to talk about your plans and make arrangements
Je ne peux pas aujourd'hui
J'ai une réunion importante
Tu es libre demain ?
To accept or turn down an invitation, say je peux (I can), or je ne peux pas (I can't). Peux is from the verb pouvoir
To say what you have to do, use je dois (I must). It's from the verb devoir and is followed by another verb in the infinitive:
With questions and negatives, English also uses "do" "don't" and "doesn't", which have no equivalent in French.