be busy doing忙于做某事，动词用成v.ing
They are busy doing their homework.
他们正忙于做作业。
be busy with sth.忙于做某事，但with后跟名词或代词
They are busy with their homework.
他们正忙于做作业。
中考语法点：be 形 doing
be worth doing 值得做某事.......
be busy doing
日日行，不怕千万里；常常做，不怕千万事。欢迎点击！！！
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