是这样,首先agreed作为形容词的时候有一个用法：
agreed = determined by common consent = in agreement
主语必须是双方的,也就是类似we are , they are agreed (sometimes on sth.)才对
这是其一.
如果你查一下任何一本权威一点的英英词典（我手头查了朗文、麦克米兰、柯林斯）,都会看到一个很明确的解释：【agreed】这个词已经成为一个感叹词,它就是一个单独成句的词了.
词条释义：
(Longman)
Agreed：used to check if someone agrees, or to show that you agree
'Let's just forget it ever happened. Agreed?' 'Agreed.'
(MacmMllan)
SPOKEN used for saying that you agree with a suggestion, or for asking people if they agree:
The last one to arrive pays for the meal. Agreed.
(HarperCollins) nterj
an expression of agreement or consent
(2)当然可以~~人家老外都这么说了还有啥不行的
(3)没有很懂.secondary?次要?
如果是同意的话,可以说
accepted ,affirmative,sanctioned