Do you know how large the earth is?请问这里为什么要加is?

how do you know How do you know? How do I know you? do you know how much i love how much do you know really about do you know how to write How many color do you know? HOW MANY COLORS DO YOU KNOW? HOW MUCH DO YOU KNOW ABOUT How much do you know about AIDS how many dinosaurs do you know? know ,you ,do ,how ,him 组成一句话 How many documentaries do you know? Do you know how to o__the machine? Do you really know how to How do you know the time?回答 do you know how old am i how did you know?与how do you know?的区别.请 在什么时候说 how do you know？和 how did you know？