1,I have finished a large part of the novel written by Dicke

1,I have finished a large part of the novel written by Dickens,the rest of which (is) very difficult.
这个is是由the rest of 决定的 所以是单数is
2,It was (he) who did it,but it is (her) that we are thinking of.
强调句,强调的就是括号里的he,而且这个he是先行词引导后面的定语从句who do it
3,Three-fourths of the surface of the earth (is) covered with water.
这个单数的is是由the earth 决定的,参见第一题
4,This is the third time whis week he has had to study late,(isn’t it)?
反义疑问句,因为前面说的是This is the third time
用的是is ,后面也要相应的用is
5,She is in a poor (state) of health,which worries her mother much.
a poor state of health,这个是固定搭配,用于说健康状况的时候就用state