句子后头有个last night,表示此句子是过去式.
英文语法中,若句子是过去式,句子里的动词就得改为过去式.所以,play是动词,就变成played,就如hear变成heard一样.
p.s.如果动词前面有一个to字,动词就必须保持原形,不能改为过去式、正在进行式等形式.（e.g.going to play the piano/refuse to play）
动词前面是not（e.g.do not、does not、did not、cannot、could not、etc.)也一样,保持动词的原形.（e.g.She does not hear、I did not hear he played his piano last night 、I could not hear him talking to me）