An insane person has a mental disorder, a foolish person may do something without thinking first. Or, insanity is a more permanent condition, while being foolish may be a one time condition.

Well, Google Translate provides 疯 and 疯癫 as translations for "insane". "insane" means "crazy". The word comes from the Latin words "in-" ("not") "sana" ("healthy") However, it is applied primarily to mental health and describes a person or behavior that is not normal to a healthy mind. Of course, it is used metaphorically to describe behavior that seems "crazy" even if the person is not actually insane. Example: It's insane to take 6 courses, play varsity sports and work part-time during college. You'll go crazy with that heavy a load. "Foolish" means to do something or act like a "fool". Google Translate provides "傻", "蠢", "笨" Example: It is foolish to ski while drunk. In many cases, the two words are almost interchangeable. However, "insane" suggests craziness, while "foolish" suggests silliness and stupidity.

Just think of 神经病；笨蛋。 but as in Chinese we often use hyperboles, just jokingly! In Chinese too to can say 'crazy, insane; That's insane! He is insanely jealous. That's a crazy price! Silly or foolish are on the one hand softer and on the other hand more critical.

I've only watched about ten episodes of "How I met your mother" and I would guess that "insane" is used in many,many episodes of that show. However, the sense I get is that "insane" is probably not likely to be used seriously in most of the dialogues. It's less likely to be used seriously in that show as "you're mentally ill and need medical help" and more light-heartedly as in "Wow! That's amazing. I can't believe you actually did that" or "That's a really off-the-wall idea. Let's do it."

Thank you for answering my question. Honestly, I'm just kind of curiosity about it. When I am watching the TV show, how I met your mother, the characters always say "insane" to describe a person. So I'm just wondering if there's any positive or negative attitude under this condition? Does "insane" sound much more like written form or something like that?