Yes, you can ... forever ...
Yes, you can use a global US forever stamp to send a letter within the United States. The global forever stamp is designed for international mail, but it can also be used for domestic letters since it covers the cost of sending mail regardless of the destination. However, it's usually better to use a regular forever stamp for domestic mail, as it is specifically intended for that purpose.
You can use US postage since origin US but since int'l will take more than 1 stamp
Yes, you can use a Forever stamp on a postcard, but it's overpaying for postage.
As of 2016, one standard forever stamp costs 49 cents.
Yes. that is their purpose, to still be valid.
Forever
No, it is not a Forever Stamp.
Yes, you can still use a Forever stamp issued by the United States Postal Service in 2009. This was the revolutionary idea of the Forever stamp - you can purchase a first-class stamp at today's price then not have to purchase additional postage later when the price goes up. The stamp is called the "Forever Stamp" because it will be valid forever for a first class envelope.
The word forever means forever. Theoretically, if the US remains in existence for several hundred years without interruption and the Postal Service continues under the US government, your stamp purchased in 2011 should be honored.
A Forever stamp is good for one ounce of First Class Postage inside the US.
You have to use US stamps in the United States, regardless of the destination.
An unused Forever Stamp is worth whatever the current postage rate is. At the moment that's 49 cents.