原句分解一下就是 the advertisement for it(为它做的广告) 做主语 is 做谓语 dishonest and harmful(不诚实和有害的)做宾语 后面如果是看成从句的话 后面还得有东西 不然就成了 the advertisement( for it is dishonest and harmful) 翻译:这个广告因为他是不诚实和有害的.后面就没了?这句话明显不完整 .所以不能当从句来理解.
没错,有定语从句,用来修饰前面的词。例如: The advertisement which is dishonest and harmful has been played a lot on TV 后面的which is dishonest and harmful 就是用来修饰The advertisement的。去掉从句句子也是完整的The advertisement has been played a lot on TV。