A monkey has learned to turn on the lights,play a recorder when she is told to.最后为什么要加to

题目

A monkey has learned to turn on the lights,play a recorder when she is told to.最后为什么要加to
A monkey has learned to turn on the lights,play a recorder when she is told to.末尾的to可以省略掉吗?tell是及物动词为啥要加上to呢?
原句为A three-year-old monkey named Helen has learned to 15 on and off the light,use a recorder and open doors and windows when he is told to.

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