I need to call my friend David ____ coat I walked off by mistake yesterday.
I主语,need to call谓语,my friend宾语,David宾语同位语,in whose coat……穿着（某人的）外套…… 做直接宾语；
in whose coat……部分可理解为：I walked off in his coat by mistake yesterday（我昨天穿着他的外套离开的.）
call sb.sth在这里理解为“呼叫（某人）某事,原句可译为：我打电话告诉我的朋友大卫,昨天走时穿错了他的外套.）为什么是 in whose coat所引导的是一个宾语从句，原句结构是这样的：I walked off in his coat by mistake yesterday