hu ran 忽然 adverb only tu ran 突然 adverb, and adjective also. chain had gave a lot of good examples. these two words have no difference in meaning.

in fact，few even same totally，everyone of them is fit

This question is very difficult even for many native Chinese speaker. It'll need a long answer to explain the difference of these two words. Here's some tips. "hu ran" and "tu ran" can both be used in unforeseen circumstances. But their parts of speech are different. "hu ran" is an adverb and can only functions as adverbial modifier in Chinese sentences. "tu ruan" is an adjective and can functions as predicate, complement, attribute, adverbial modifier in Chinese sentences. That is to say, they have the same meaning when they function as adverbial modifier. For example, 美国突然中断了对阿富汗的援助。 美国忽然中断了对阿富汗的援助。 And they also have many differences. For instance, We say 这件事很突然。 but we don't say 这件事很忽然。 We say 事情发生了突然的变化。 but we don't say 事情发生了忽然的变化。 I don't know if you could read Chinese characters, if the explainations are difficult for you to understand, just remeber you can use "tu ran" under all circumstances, but "hu ran" is not. And "hu ran" can be replaced by "tu ran".