关于v.的单三形式：
○1通常在动词后加s.如 work→works
○2以s,x,ch,sh结尾的动词加es.如 teach→teaches
○3以辅音字母加y结尾的动词,变y为i,再加es.如 study→studies
○4以o结尾的动词加es.如 go→goes
○5 have → has