See to it that all the light (are) turned off before you leave
See to it留意必定做…
that 定从
it=all the light (are) turned off before you leave all the light 所有的灯 turn off 关闭
before you leave在你们离开(出发)前 are+turned表被动语态
should be应该是 虚拟语气 请看以下对should be的用法：
shall,是用于第一人称的,即I和we,在建议第二人称的人（you)时,should.而且should,也是shall的过去式
例：你应该去看看医生—You should go to see a doctor.
我们应该坐下—we should sit down.
通常用should比用shall多,而且should的语气比较委婉.
为什么写are?为什么 should be不行?你动动脑子,是不是明白了?