3, When the article arrived, the editor read the first sentence and then refused to publish it.

When the article arrived = When the editor received the article

refuse to publish it

区别：refuse: vt. 拒绝（态度严厉） e.g. I invited him to dinner, but he refused it.

decline: vt. 婉言谢绝

repudiate: vt. 断然拒绝

4，The article began: 'Hundreds of steps lead to the high wall which surrounds the president's palace.'

which 指代the high wall

5，The editor at once sent the journalist a telegram instructing him to find out the exact number of steps and the height of the wall.

instructing 现在分词进一步补充说明fax的情况

6, The journalist immediately set out to obtain these important facts, but he took a long time to send them.

set out to do sth. 决定，打算，着手做

相当于 decide and try to do

set about doing

take sb. a long time to do sth. 某事花了某人很久的时间

e.g. It took me five days to write the article.

7, He sent the journalist two more faxes, but received no reply.

two more faxes = another two faxes

more表示"又，再"