1．At a young age, he (move) to Spain and now plays professionally for the FC Barcelona.
moved 解析：根据时间状语At a young age可知，此处表示过去发生的事情，故用一般过去时。
2．It (rain) at that time and the streets were full of holes which were full of water.
was raining 解析：根据时间状语at that time可知，当时正在下雨，即“下雨”这一动作在过去某段时间内正在进行。故用过去进行时。
3．I just graduated from West Coast University. I (receive) job skill training just before that, but I had never worked.
had received 解析：此处根据just before that可知，我在之前受过职业技能的培训，根据上句中的一般过去时可知，此处应用过去完成时。
4．The full railway, which includes 72kilometer section through Beijing, (complete) in four and a half years.
will be completed 解析：句意：这一整条铁路，其中有72千米经过北京，将在四年半内完工。设空处作主句的谓语，根据时间状语in four and a half years可知，应用一般将来时；且主语The full railway与complete之间为被动关系，应用被动语态。
5．We (reduce) emission of air pollutants in recent years, but cars are still major source of them.
have reduced/have been reducing 解析：由句中的时间状语in recent years可知，应用现在完成时或现在完成进行时。
