根据句意，在答卷标有题号的横线上，写出括号内所给单词的适当形式。
(1)The letter failed to be posted because he wrote the address (correct).
(2)Did his parents buy (they) a big cake for their twenty years of marriage?
(3)The old man couldn't be (glad) to know that he finally passed the driving test.”
(4)Though I have read about three (four) of the novel, I still can't guess the ending of it.
