因为说这句话的时候sth已经发生了,要用完成时,并且,如果你用happened,是过去式,而你前面的句子i don't know是一般现在时,前后矛盾的.补：如果前面是I didn't know,那句子就是I didn't know what had happened to him.my god，还有这种说法