"לעולם לא" vs "מעולם לא"
Are these entirely interchangeable? I haven't been able to figure out when one is supposed to be used over the other. They both seem to mean never.
Also, are the non-negated forms ever used?
I think the "לעולם לא" is about the future events/actions (e.g. "Don't ever do this again."), while the "מעולם לא" is about the past (e.g. "I have never done this.").
Take it with a grain of salt though, I am not a native speaker.
While Michael's answer is correct, the short version is לעולם לא = ever מעולם לא = never
Non negated forms can be used, however they are rarely used in daily speech. It sounds a bot dramatic/poetic... You could say somethink like: "אני דן אותך לעולם! " Which would be 'I condemn you forever'. "אני אוהב אותך לעולם" Etc. Also you could say "אני מעולם אמרתי שהיא יפה" - I have always said she's beautiful. Still, these forms are not usually used. Non negated forms are used more casually as part of an expression מאז ומעולם Or לעולם ועד. The whole expression has the same meaning, it just serves for emphasis.