Hard is a simple word, means something is solid or not easy. Difficult only means not easy, it's more specific, and is casual/formal Both words are equally popular, so you can use them interchangeably. They both mean the same degree of being not easy.

Look at the following link: http://dictionary.reference.com/browse/hard You will find that the word 'hard' has 41 adjective meanings/definitions, 9 adverb meanings/definitions, and 3 noun meanings. So, I decided not to post them all here. You can find the defintion or meaning of 'difficult' by searching for the word at the same website: http://dictionary.reference.com/browse/difficult You can also use the Thesaurus to find synonyms for both words at: http://dictionary.reference.com/ After reading the defintions for both words it will appear that both words sometimes mean 'not easy to do' .... therefore 'hard' = 'difficult'. It's true that they are used interchangeably, but I believe that it would be better to use "difficult" to describe things that are 'not easy to do' and use 'hard' to descrbe things that are 'solid, not soft'. I would be inclined to use 'difficult' when something is difficult, like trying to keep my long-winded explanation brief - for example:)

The onliest difference is, you cant use the word "difficult" if you speak about the condition of an object.

Nothing. Except that "hard" has other, interesting meanings.