[译] 那些男孩子正忙着预备功课。
[误] The boys were employed to prepare their lessons.

[正] The boys were employed in prepare their lessons.
[注] "忙于某事"的英语句型可用"be employed in doing sth."="be busy(in) doing sth.",而不用"be employed to do sth."。be employed to do sth.表示"被雇佣来做某事"。