what causes gingerbread cookies to crack and beome overly hard while baking?

Too high heat, and leaving them in too long. Unless cookies call for a lower temp, you should never bake a cookie above 325. No matter what. Mrs. Fields swears by this and we know how successful her cookies are. Anyway, use a timer and figure out the perfect amount of time for your oven. Also remember that cookies will continue to bake when taken out!

I rotate my sheets 1/2 during baking, switch the top with the bottom and turn them a 180 degrees too.

Also have 4 cookies sheets, 2 in the oven and 2 out, once the ones that are out are cook prepare them with your next batch. But never, ever put dough on a hot or even a warm cookie sheet.

To high of heat or you leave them in to long. If you live in a dry area try putting a pan and water on the lower shelf.

these cookies are supposed to crack.just adjust your time so they don't over cook