I walked in our garden,——Tom and Jim were tying a big sign o

定语从句关系词（关系代词,关系副词）的考察.选择哪一个关系词关键看先行词在从句中做什么成分,也就是“缺啥补啥”.通过观察从句中缺少地点状语,而主句中的garden这一先行词,恰好符合这一成分.至于你说从句中没有in 你说的很对,这也是关系词为什么要用where的原因.如果此句改成 I walked in our garden, ____ tom and Jim were tying a big sign onto one of the trees in. 那么答案可以选 which.即 where = in which = in the garden 做地点状语.