Todo is all, whole, entire, complete or every. Examples: todo dia (every day), li o livro todo (i read the whole/entire book), passo o dia todo trabalhando (i spend all the day working), on the other hand, tudo means everything, all. Tudo certo? (all right?), eu sei de tudo (i know everything), tudo ou nada (neck or nothing), antes de tudo (first of all)