✔ 最佳答案
The "subject" in "is subject to" is an adjective, it means "likely or prone to be affected by" (易受……的).
Hence
"The boy is subject to habitual headache" is the correct one.
The "subject" in "is subjected to" is a verb in a passive voice (被動語態), it means "be forced to undergo" (受制...).
Hence
"We should be subjected to law" is the correct one.
2014-04-12 22:25:55 補充:
For you NOT to get confused, you shall pay attention to the function of "subject" after "be-verb". If it functions as a verb, being placed after be-verb, it needs to conjugate (動詞的詞形變化). If not, it will function as an adjective.