it is (what) the pubis do in their spare time (what)really prepares them to take their place in society as citizens when they grow up>
疑问：
这句话连用两个what，它们的意义是什么？第二个可不可用that代替？

1)第一个what是表语从句the pubis do in their spare time 的宾语同时也是从句的先行词；第二个what是真正的主语从句(what)really prepares them to take their place in society as citizens when they grow up的主语，it是形式主语。
2）第二个不可用that代替。因为第二个what即指第一个what的表语从句。

原句正常语序是：what really prepares them to take their place in society as citizens when they grow up is what the public do in their spare time.

经转换为：what the public do in their spare time really prepares them to take their place in society as citizens when they grow up。


大意：未成年人的业余活动的确为他们长大成人后作为社会中的公民各尽其位作好了准备。

the public：在此指未成年人。