Hello Ameen, the main difference is that "in dem" or "im" is a prepositional phrase, where as "indem" (notice that it is written as one word) is a conjunction. "in dem" or "im" means "in the", where as "Indem" means by, by doing this" For example. Die Katze ist in dem Haus - The cat is in the house. Ich lerne Deutsch, indem ich viel übe - I learn German by practicing a lot. Hope this helps :)