first, the two words have different spelling and pronunciation， “还” [hai] “在” [zai] anyone of the two words can be combined with the other words, for example: 还有 -> means : also, still. 还是 -> means : still , all the same etc. of course,we have a combination word like this 还在！ 为什么你还在这里？ -> why are you here then? 你还在等什么？-> what are you waiting for? i will write down some sentences about them. *还 （1）return back 请把书还给我 -> return the book back to me please. （2）yet 她还没意识到她的错误。 -> she did not realize her mistake yet. *在 in, on 他曾经在上海住过。 -> He once lived in shanghai. 在这点上我们看法不同。 -> We differ with them on this point. --------------------------------------------- If anyone have any question about learning mandarin or cantonese, please let me know, it's nice to share knowledge.

they're totally different 还 is an adverb meaning yet/still 在 is a verb or preposition indicating existence