句子中为什么用was This wasn't how it was supposed to happen 事情本不该是这

e supposed to
be supposed to 的用法
用法一：be supposed to...其中to是动词不定式符号,不是介词,其后要跟动词原形.当be supposed to...的主语是“人” 时,意为“应该…… ”；“被期望……”,它可以用来表示劝告、建议、义务、责任等,相当于情态动词should.如：
Everyone is supposed to wear a seat-belt in the car.
每个人在汽车里都应该系安全带.
Teachers are supposed to treat all the students alike.
老师应该对所有的学生一视同仁.
用法二：当be supposed to...的主语是“物”时,它表示“本应；本该”,用于表示“某事本应该发生而没有发生”.如：
The new laws are supposed to prevent crime.这些新法令本应该起到防止犯罪的作用.
The train was supposed to arrive half an hour ago.火车本应在半小时之前到达.
用法三：be supposed to后面接“have + 过去分词”时,表示“本应该做某事而没做”.如：
You are supposed to have handed in your homework by now.
现在你应该已经把作业交上来了.
He is supposed to have arrived an hour ago.他应该一小时前就到了.
用法四：be supposed to...的否定结构为be not supposed
to...,它常用于口语中,意为“不被许可；不应当”.如：
She was not supposed to be angry about that.她本不该为那件事而生气的.
You are not supposed to smoke on the bus.你不应该在公共汽车上吸烟.