French negates a verb by wrapping it in two parts: ne before the verb and pas after it — je ne sais pas (I don't know), il ne parle pas (he doesn't speak). With a compound tense like the passé composé, pas goes right after the auxiliary, not after the past participle: je n'ai pas mangé, not je n'ai mangé pas.
In casual spoken French, the ne is very frequently dropped altogether, leaving just pas after the verb: je sais pas, c'est pas grave. This is extremely common in everyday conversation, but ne is still required in writing and in any careful or formal speech.