What is the difference between 突然 and 忽然 ?Feel free to just provide example sentences.

it's similar,but sometimes quite different.According to 《现代汉语虚词词典》
突然：表示情况在短促时间里发生，使人出乎意外，用在句子开头时后面有停顿，后面加个逗号。    
忽然：表示情况迅速发生，出乎意料。
1.忽然as a adv. can swap the word突然。but 突然is emphasize more on the situation happened rapidly than 忽然 do.
2.忽然is rarely used before the subject.
More 突然is an adj ，忽然 is an adv，突然can be followed by a noun. ex：“突然情况”“突然事故”“突然变化”。突然 can also be a predicate，there is often a "很" or "不" modifier in front of it.Although 突然 is a adj，it still have a function to be a adv.It can be used before the subject，ex“突然电话响了”“Suddenly the phone rang” ，you can突然 in front of “不+v.” “没有+n.” like“突然不响了”“突然没有响了”，but you can not use 突然 in front of “没有+v.”，like“突然他没有吃了”is wrong，“突然他不吃了”“suddenly he stops eating”is correct.No matter being a Adverbial or a predicate the meaning of the 突然 is the same.
For the 忽然 the 词性（the Part of Speech？？？）is fixed.a adv. like “忽然刮起了大风”“A strong wind suddenly blew up”
Sometimes both can be used in general.In a unexpected situation.
1.我正要出去,忽然(突然)下起了一阵大雨。   2.我的自行车刚才还在这儿,怎么忽然(突然)不见了。3.忽然(突然)停电了。
Sometimes we use 突然 to describe a objective events“地震突然发生了”，“他突然死了” “他的病来的那么突然”，“事故发生的那么突然”
i just translate that from ，not all of it ，but hopefully enough...
https://wenku.baidu.com/view/ec7da2680875f46527...