“I was told I could board the airplane subject to a security scan.”
“At the airport I was subjected to a humiliating search.”
Does it help you to distinguish between these expressions to know that “subject” in the first example is an adverb and “subjected” in the second example is a verb? Didn’t think so.
Although these two expressions can sometimes be switched with only a slight change in meaning, they are not equivalent. To be subjected to some sort of treatment is to actually be treated in that way, usually in an objectionable way.
But to be subject to a regulation, to taxes, to discussion, to inspection, to any sort of condition, is to be liable to it. In some contexts, the conditional action is mandatory: “Shipment will be made subject to approval of your charge card.” In others, the conditional action may be theoretical, not uniformly enforced: “This Web page is subject to change.” Many people mistakenly use “subjected to” in this sort of context.