My family owns a Christmas tree farm so here are a few tips:
Do not put sugar in the water, it does nothing.
Yes, keep it watered. If it ran out of water, the sap will have created a seal on the bottom of the tree and no matter how much water you put in it, it will not drink anymore.
If you bought a precut tree, then the tree probably came from Canada and was cut in September so it is probably dead anyway.
Be careful now that it is drying out!!!!