How to get an item to place a block in inventory

I'm making a pretty small mod, I've only got 14 items planned (2 sets of armor, one block, four swords, and a special weapon). Each of the swords has a special ability. I'm having a lot of trouble with the second sword in the set. It's supposed to be able to place whatever block is in the slot immediately to its right in the hotbar, but I can't figure out how to get the procedure to do that. FYI, this is my first mod and I am totally new to this software. I have a decent idea, but no clue what I'm doing. Any help would be appreciated. Thanks!