Had you adventured when you were with friends, it is fun. ってのだと、仮定法のHadかな?ってのはわかるんですが、 "is Had"ってのがいまいちよくわからなかったりします。 倒置して、you were を省略して、 It is fun, Had you adventured when (you were) with friends. さらに強調のために Fun it is, Had you adventured when with friends. んで、itが自明だから、省略? Fun (it) is, Had you adventured when with friends. ってので、カンマが消える? Fun is Had you adventured when with friends. んで、whenも移動? Fun