He is rehearsing his lines for the school play.

rehearsing 是动词 rehearse 的现在分词形式。rehearse 由前缀 re-（表示"再次"）和 hearse（原指"犁沟"，引申为"反复练习"）组成，意思是"排练、彩排"，指为正式表演而进行的练习。