sing along指“跟着某人一起唱”,是个不及物的动词短语,如Sing along if you know the words.如果你们知道歌词,就跟着一起唱吧.至于它与其他介词或副词再搭配,那就看语境了.如你说的sing along on可能是sing along on a special occasion,或on表示“持续”,也就是“一起一直唱下去”；而sing along to可能是“伴随着.一起唱”,如伴随音乐、节奏等sing along to the music/rhythm等