They [ s [补充单词]]in the winter .

They [ swim ]in the winter .
他们在冬天游泳。
What are they doing？They are[ swimming ]。
他们在做什么？他们在游冬泳。

They [skate滑冰]in the winter .
What are they doing?
They are[skating ]。

I-am-was
you -are-were
we-are-were
they -are-were
he-is -was
she -is -was
it-is-was

楼上的问题回答不对啊.

1、They [ s [补充单词]]in the winter

这个有很多答案,合理就可以了啊,比如说skate,swim,sing等等都可以啊.

2、What are they？They are[ ]

这个题是问他们是干什么的？是问职业，不是问他们在干什么啊，在初中都学过，所以

They are teachers/soldiers/workers/engineers/doctors/firemen/repairmen/drivers/accounters/students/polices/officers/CEOs.