Can I purchase anything in the US with Euros?
No, money from other countries, even Canada and Mexico are not legal tender in the US. You need to go to a bank or an Exchange and have it exchanged for US money
No, foreign money cannot be used as currency in U.S. You can take it to a bank for exchange, however you'd have to have enough of it make it worthwhile as the fee can be pricey. Best to exchange at the airport currency counter.
No foreign money is not in valid in the us. You must trade the foreign money for U.S. money.