Nie and niemals are often used as synonyms. Duden, the bible of German grammar, says that niemals is used instaed of nie when you want to stress or assert your statement. The English translation of these terms points in a similar direction: https://dict.leo.org/englisch-deutsch/niemals niemals: not at any time, under no circumstances, while these translations do not appear for nie. But there are some expressions where nie just sounds more idiomatic. ◦er hat sie nie verstanden ◦nie mehr! ◦nie wieder Krieg!