I'm frequently encountering English proofreading problems. For now I am trying to figure out which one of these two is the correct form: in purpose or on purpose. I tried to Google and found both forms used in various web-pages. If both of these forms are correct, please explain when which one is better to use.
on purpose is what's used. This is a legitimate expression in English with its own entry in the dictionary. It means intentionally. I myself use it all the time. As for the other one, I have never ever heard anyone say in purpose in place of on purpose. That would just sound wrong.
However, you could say in purpose in certain situations, but that would mean a totally different thing. For example:
This thing is completely useless in purpose as well as in use.
Meaning, it for whatever reason does not have a purpose and thus is considered useless.