你要首先分清主句和从句,然后看关系代词在从句中所做的成分就可以了.(你要不知道主谓宾的区分就要努力补习下了...)
1)The boys (who are playing football) are from Class One.
括号中为从句,单拿出来,who are playing football,who 指代the boys,(the boys )are playing football,在其中做主语.
2)Yesterday I helped an old man (who lost his way).
同上,who在其中做主语.
3)This is the pen ((which) he bought )yesterday.
which在其中相当于the pen,he bought (the pen),做宾语,所以可以用which或省略.
好好学习英语那~