分析（语法结构、句意等各方面）Which of the two cows ___that___ you keep produces more milk?

句意：你养的两只奶牛里面哪只产奶多？

Which of the two cows （that you keep） produces more milk?

that you keep是cows的定于从句，你养的奶牛。去掉以后句子仍然是完整的。

Which of the two cows that you keep produces more milk?

主语是：Which of the two cows that you keep （其中that you keep 是定语从句，修饰cows ）

谓语是：produces（produce加s是因为which指其中的某一只奶牛，是单数）

宾语是：more milk（加more是因为两只奶牛产奶作比较）

句子意思是：你养的两只奶牛，哪只产奶更多？

which of the two cows that (you keep) produce more milk?是produce,两只牛所以不加s.
你养的哪两只奶牛产更多的牛奶.
that 代指那两只牛,you keep也是修饰那两只牛的
which of the two cows that produce more milk?
这样就清楚了些.

你养的两只奶牛，哪只产奶更多？
that you keep是定语从句，修饰cows