1.A plane is a machine that/which can fly.2.The boy,whom you punished yesterday,was lost .3.These are the pictures that/which were bought from the western countries.4.Do you see the bridge which/that was built last year?找准两句话中的相同部分,根据在句中所指代的人或事物决定使用that、which还是who、whom.1、3、4句中的引导词都指物,且在句中作主语,因此用that/which都可以.第2句指人、且在句中作宾语,因此只能用whom.