可以这么翻译:The boy noticed a girl in red clothes who was running across the street last week.
这句话的翻译思路是:
The boy noticed a girl: 描述男孩注意到了一个女孩。
in red clothes: 描述女孩穿着红色的衣服,这里用了介词短语作后置定语。
who was running across the street: 描述女孩上周在街道上跑着穿过了马路,这里用了关系代词who引导定语从句,表示穿过了街道。
last week: 表示时间为上周。