hear sb do sth 听到某人做某事,暗示过程完成.如：我听到他关掉电视.I heard him turn off the TV.
hear sb doing sth 听到某人正在做某事,强调动作的进行,可能你听到的只是动作的一部分.如：我经过这里时听到有人唱歌.I heard someone singing when I passed by.
如果用I heard someone sing the song.就暗示“我”听完了整首歌.