为什么是be busy doing而不是be busy to do ?

be busy with doing sth or by busy with sth 这里的with省略了，习惯用法

be busy doing 是固定词组的用法：相似的还有：be worth doing

be busy with doing sth 与 be busy with sth 这里的with省略了，英语中习惯用法

be busy doing 实际是doing前省略了个with

be busy （in） doing sth，习惯表达

.....外国人这么写啊