原句：At present，it is very common that marriage across the nations
修正：At present，it is very common to find marriage across the nations.
原句：If they from different countries，they will have different living habits.
修正：If they are from different countries，they will have different living habits.
原句：If continue，there is no room and no resources left for more people in the future.
修正：If this continues，there is no room and no resources left for more people in the future.
原句：To finish the work in time，the machine is kept running all the time.
修正：To finish the work in time，we kept the machine running all the time.
原句：Looking out of the window，the grassland stretch as far as the eye can reach.
修正：Looking out of the window，I can see the grassland stretch as far as the eye can reach.
原句：Having heard the terrible news，his heart beat fast．
修正：When he heard the terrible news，his heart beat fast．
原句：At the age of ten，my grandmother died．
修正：When I was at ten，my grandmother died．
原句：To get ready for the trip，all the things she needed were put into a suitcase．
修正：To get ready for the trip，she put all the things she needed into a suitcase．
原句：“Six chairs” are not correct here．
修正：“Six chairs” is not correct here．(如果主语为引语，谓语动词要用单数。)
原句：Politics are more difficult than physics．
修正：Politics is more difficult than physics．
原句：Whether or not we'll go to Beijing have not been decided yet．
修正：Whether or not we'll go to Beijing has not been decided yet．
原句：Sometimes the wealthy also needs help．
修正：Sometimes the wealthy also need help．
原句：When I was in high school，you never knew what to expect．
修正：When I was in high school，I never knew what to expect．
原句：I was extremely sorry you are injured．
修正：I am extremely sorry you are injured．
原句：In high school we worked more as a group than as an individual．
修正：In high school we worked more as groups than as individuals．
原句：His job was a policeman．
修正：He was a policeman．
原句：Before I selected the new course，my teacher warned me of the difficulty of the course and of its length．
修正：Before I selected the new course，my teacher warned me how difficult the course was and how long it lasted．
原句：We asked the store manager to show us a gift that was inexpensive，useful and looked nice．
修正：We asked the store manager to show us a gift that was inexpensive，useful and nice looking．