be busy with 忙于某事be busy doing sth 正忙于做某事前者是一般时态，后者是强调进行时态
be busy doing sth侧重于动作 sth 是名词哦· do是动词的形式be busy with sth 侧重于状态意思都一样,可以互换.He is busy doing homework.=He is busy with homework
He pretended to be busy with some work .他假装忙着什么事means to be busy doing something.
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