They heard a message from a spacecraft.(用just改为现在完成时态形式)
They        just        a message from a spacecraft.
Have you got a letter from Lucy?(改为同义句)
Have you                Lucy?
To understand the problem is very difficult.(用it作为形式主语)
               very difficult        understand the problem.
I came here two days ago.(改为同义句)
I                here         two days.
Her mother has been to Hong Kong several times.(就画线部分提问)
                        has her mother been to Hong Kong?