竟然：means out of some one logical thingking and outwith reason； 忽然，突然，words whose mean is similar.... 忽然means suddenly (adv)，突然means sudden(adj).突然Said the situation happened faster and more urgent than忽然 他竟然是个小偷。 他突然生病了。 他忽然来了。

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竟然：means to one's supersise;for example:he speaks so fluently,which out of my imagination. 忽然 is similar to 突然，means "suddenly".他“竟然”是英国人!本来天气很好的，”突然“下雨了，真是没想到。安静的马路上”忽然“来了辆车，吓人一跳。

忽然 和 突然 很相似 ( = suddenly)，在汉语中很少区分这两个词语 (We don't usually distinguish them)。它们和时间相关。 竟然 ( = surprisingly) 和时间不相关，仅仅表示惊讶的意思。

竟然：surprise，you cannot think about it 忽然 and 突然 are both mean something happend very fast and surprised，but 突然 is faster than 忽然