The teacher told them since light travels fast than sound ,l

应该是lightning,名词,闪电的意思.
appear to: 看起来像
The teacher told them since light travels faster than sound, lightning appears to go before thunder.
老师告诉我们因为光传播得比声音快,所以看上去闪电出现在雷声之前.
再问： go出现？
再答： 你要直译的话就是闪电走在雷声前面。go就是本意，走。意译出现更恰当。