What to do with "extra" Class cards

[Muse23PT]

Hi again friends,

I started in the world of Munchkin about 2 months ago, and received my Munchkin Deluxe box last week and It came without the yellow pawn in the sealed bag :(

But haven't had the time to play many games, and when I do it's usually with people that I'm introducing to the game, so many questions are posed during a game... In today's game these were the questions:

1- Race and Class (and I guess Mojo for Munchkin Zombies) that I don't have in play because I already have one of each in play can be put down sideways on the table indicating that they are not in use? Or do they have to be in the hand?

2- Related to the first question, does it work the same way for Powers in Zombies? I ask because you can have as many powers in play as long as that there combined rank isn't greater than your level, so can we put down Powers sideways on the table and activate them when we reach the level that allows us to do so? Or do we have to keep the Powers in the hand and only play them when we can use them?
And what if I'm using Powers equal to my level and I lose a level, do I put the power card that I choose not to use to respect the power rank rule sideways and then activate the power again when I gain a level or do I have to discard it?

3- The same question for when a curse is played that I have to lose my Race or Class, the items that are used only by that Race/Class are put sideways or do they get discarded?