49．In the 1500s when the Spanish moved into what later was to becomethe southwestern United States，they encountered the ancestors of the modern day Pueblo，Hopi，and Zuni peoples．

(when引导定语从句，what引导的从句作movedinto的宾语)

16世纪西班牙人进入后来成为美国西南部的地方时，他们遇到了今天皮艾布鲁人、霍比人和株尼人的祖先。

分句1：in the 1500s…

分句2：when…

分句3：what…

分句4：they encountered…

2、3和1嵌套

1和4并列