1.Such training was common in acting families at this time,especially when the family income was often uncertain.请问这个句子中

1.Such training was common in acting families at this time,especially when the family income was often uncertain.请问这个句子中为什么“acting”是动名词?
答：这样的训练在当时的表演家庭中是很常见的,尤其是当家庭收入经常不稳定的时时候.
acting有可能是由动词act变来的非谓语动词,是现在分词还是动名词,要经过思考.
也有可能是形容词 如：the acting President
还有可能本身就是一个名词:the art of representing a character, esp. on a stage or for a film.
从a sleeping car = a car for sleeping 知,动名词作定语表示“用途”
而a sleeping boy= a boy who is sleeping 知,现在分词作定语表示“主动并（或）进行”.
这里我看不用是用途（acting families families for acting)
更象是主动：acting families= families who act (表演的家庭）,所以我认为是现在分词.
现在的语法学家没这么细心,只说动词的-ing 形式.
2.There is something in the box.请问为什么这个句子中“in the box”做的是补语?
答：there is sth. in some place.中的介词短语in some place是作地点状语的.表示“在.地方”有某物.
there is sb./ sth. +介词短语,是一个句型,与主系表句型不是同一个.
不能说这个介词短语是补语,因为有时候可以没有这个短语,句子仍成立.
且there be...= there exists...,be表示“存在”,就如To be or not to be中的be一样,不是系动词.
所以一般说它是地点状语,如果是there is a film in the evening.则为时间状语.
问题补充：
哦哦~不好意思~第二个句子我记错了.应该是为什么These things should be kept in the box.中的“in the box”做的是补语?
答：是的,是主语的补足语.主动语态是：We should keep these things in the box.
这句话中的in the box显然不是说“我们应该在箱子里去做某事”,也就是说,in the box不是用来修饰谓语动词keep的地点状语,而是作宾语these things的补足语的.宾语和宾补在逻辑上有主表关系：These things are in the box.
当主动句中的宾语变成被动句的主语时,其补足语也有宾语补足语上升为主语补足语了.