A monkey has learned to turn on the lights,play a recorder w

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.