when：in/at/on+which(时间点）
where：at/in..介词+which（place）
why：for +which（ the reason）
关键还是要看语意和先行词是什么,用什么介词.能告诉我区别吗when = 在什么时候，where = 在什么地方，why = 为什么原因，你看先行词是什么，分析分析句子的意思。比如：Tell him to go to the classroom where we often have our English class 这句话。classroom就是先行词，后面你可以看成 we often have our English class in the classroom。in the classroom=in which=wherePeople will always remember the time when Hong Kong and Macao returned to our motherland .Hong Kong and Macao returned to our motherland+in the time 。in the time=when恩...我今年高三，这些就是我们老师平时上课讲的。你不懂还可以追问噢~亲~~把分给我吧~~谢谢咯~~~~>.<~~~