77．The achievements of the colonial artists，particularly those of Copley，West, and Peale，lent credence to the boast that thenew nation was capable of encouraging genius and that political liberty wascongenial to the development of taste——a necessary step before art could assume an important role in thenew republic．

(The achievements…lent credence to the boast that…and that…为句子主干，两个that引导的两个从句都作boast的同位语)

这些殖民地艺术家的成就，尤其是考伯莱、韦斯特和皮尔的成就，为新国家有能力鼓励天才和政治自由与艺术品味的发展相一致的宣言提供了凭证——这是艺术，在新的共和国里担当重要角色之前所必须经历的一步。

解析：

主句：Theachievements of the colonial artists lent credence to the boast

同位语从句1：that the new nation was capable ofencouraging genius

同位语从句2：that political liberty wascongenial to the development of taste 其中

“——“引导的从句：anecessary step before art could assume an important role in the new republic．

修饰成分：

插入语：，particularlythose of Copley，West, andPeale，

其中两个同位语从句1和2由and并列在一起，共同修饰主句中的宾语boast

“——“引导的从句是用来修饰整个主句的，对其进行补充描述

插入语修饰artists