当父亲看见幼年莫扎特 (Mozart) 毫无困难地演奏乐曲时，他恍然大悟，儿子身上蕴藏非凡的音乐天赋。(occur)

When the father saw young Mozart playing the music effortlessly, it occurred to him that his son possessed extraordinary musical talent.

考查时态、从句和固定句型。根据汉语提示可知，本句为when引导的时间状语从句，陈述过去的动作，为一般过去时。“父亲” 为the father；“看到” 为see，“看到某人正在做某事” 为see sb. doing sth.；“毫无困难地演奏乐曲” 为play the music effortlessly；“恍然大悟” 为it occurred to sb. that；“他的儿子” 为his son；“蕴藏” 为possess；“非凡的音乐天赋” 为extraordinary musical talent。故翻译为：“When the father saw young Mozart playing the music effortlessly, it occurred to him that his son possessed extraordinary musical talent.”。