Subject to is an adjective
Subjected to is a verb
if something is 'subject to' something it means they are affected by that thing / prone to get that thing... e.g. He is subject to headaches - implies he gets them often.
If someone is 'subjected to' something - this implies that someone's actions caused you to experience something (usually something unpleasant).
E.g. I was subjected to two hours of questioning (meaning someone questioned me for 2 hours and it wasn't a nice experience). I was questioned for two hours doesn't mean it was an unpleasant experience necessarily. adding 'subjected to' adds more weight to it being unpleasant.
Something can be 'subject to' something else (a condition) - e.g. The sale of this house is subject to the buyer paying a deposit by noon tomorrow (ie, if they don't make a deposit then the sale cannot continue).
I hope this is of help.