Can you spend a US silver certificate?

Answer 1

Yes, but it may not make a lot of financial sense to do that. With the general exception of very common bills like 1935 H and all 1957 $1 silver certificates, most of them are worth more than face value to a collector so you'd make a (possibly small) profit selling it instead of spending it.

Silver certificates were never withdrawn or demonetized so as far as the Treasury is concerned they're still legal tender. You also could have some difficulty spending one, though, because many cashiers have never seen a silver certificate and automated bill acceptors generally don't recognize them.

The only thing you can't do is exchange them for silver metal. The Treasury had to halt that practice back in the 1960s when the price of silver was deregulated.