42．Add to this the timidity with which unschooled artisans—originally trained as stonemasons，carpenters，or cabinetmakers--attacked the medium from which they were to make their images，and one understands more fully the development of sculpture made in the United States in the late eighteenth century．

(add to this the timidity…=add the timidity…to this为祈使句，相当于一个条件句：if one adds to this the timidity…)
这个再加上没有受过学校教育的艺人——他们最初是被训练为石匠、木匠或家具木工的——对待他们要制作成雕像的媒介物的胆怯，使人更充分地了解18世纪晚期美国雕刻艺术的发展。

分句1：Add to this the timidity
分句2：with which unschooled artisans—originally trained as stonemasons，carpenters，or cabinetmakers--attacked the medium
分句3：from which they were to make their images
分句4：and one understands more fully the development of sculpture made in the United States in the late eighteenth century．
分句4是整个长句子的主句
分句1、2、3是分句4的条件从句，证据意为：“把这些结合起来，你就会明白……”
分句2修饰timidity，虽然这句的长度最大，信息最多，但仍然只是整句话最终结论的一个支撑。
分句3是修饰medium的定语从句
1和2嵌套
2和3嵌套
123 和4嵌套