NEGATIVES IN PORTUGUESE:

There are two basic principles governing the use of negatives in Portuguese:
- unlike English, double (or even triple) negatives are the norm;
- when a negative word (nada, ninguém, nunca, jamais, nem, nenhum) occurs after the verb, the verb must be preceded by "não" or another negative word. Examples:

Não vi nada.
I didn’t see anything. / I saw nothing.

Eu nunca tive nenhum problema.
I’ve never had any problem.

Ref.: Modern Brazilian Portuguese Grammar - A Practical Guide.