Can we still use the 42 cent stamp?
Yes, as long as you get another 13 cent stamp to go with it.
You can always use your 42 set stamps if they raise the price that it takes to mail a letter just a fix a couple of Penny stamps to it
Yes, If it says forever, it's good for one ounce, first class. if it says 42 cents, you'll have to add thirteen cents postage
Of course you can "still" use the 42 cent stamp, you can still use a 3 cent stamp if you have one.