按要求写出单词的变化形式
lead（过去式）__________             
impossible（反义词）____________     
perhaps（同义词）______________   
encourage（名词）____________     
cycle（名词）___________          
certain（副词）_____________
time（形容词）____________         
slap（现在分词）_______________
space（同义词）____________        
sell（名词）_______________