I shouldn't be playing with his things.
翻译是：我真的不应该玩他的东西。（虚拟语气，现在时表示与现在事实相反的假设）
should 出现在“ should be doing sth ”或“ should have been doing sth ”等句式中,表示“应该正在……”或表示说话人的某种不满情绪。
例如：Why should we be sitting here doing nothing?我们干嘛坐在这里闲着?
You should be wearing a mask.你应当戴着口罩.
