He often wrote them down on small cards. 首先,按词性判断：he是人称代词的主格,考虑做主语；wrote是动词,考虑作谓语；on是介词,可以与名词连用构成状语,且所给词中有small、cards,可以构成地点状语on small cards,又因为down作副词可以与所给词wrote搭配构成动词短语write down 且因为是动副搭配,所以当宾语是代词时放中间,所给词them恰为人称代词的宾格,做宾语,且放在wrote与dowm之间,所以连词成句组成句子He often wrote them down on small cards.（他常把它们写在小卡片上）
