It is useful to be able to predict the extent to which a pri

it:形式主语；
is系动词,作谓语的一部分；
useful表语,作谓语的一部分；
to be able to predict the extent to which a price change will affect supply：真正的主语.
真正的主语是一个不定式短语,其中the extent to which a price change will affect suppl是predict的宾语,这个宾以extent为中心词,to which a price change will affect supply是定语从句,修饰extent.
在这个定语从句中,a price change是主语,will affect是谓语；supply是宾语.