No. You have to add a 1-cent stamp for a first-class letter because the postage is now 42 cents.
U.S., first class, 1 ounce or less: 44 cents.
Yes, you can still use the stamp. You have to add 3 cents to it to make up the current 44 cent rate for one ounce of mail.
More than likely, the post office will stamp "Insufficient Postage" on it and return it to you. Not only will you lose out on your 39-cent stamp, but you will also lose the time of delivery. It would...
1, but that depends on the weight of the package. If its just a normal hand wrote letter one stamp will be fine.