Hū rán (忽然) and tū rán (突然) both express the notion that something or some action takes place suddenly and unexpectedly in a very short lapse of time. They are interchangeable when serving as adverbs, the subtle different is that the semantic meaning of tū rán (突然) is deeper in the degree of suddenness than hū rán (忽然).
wài miàn hū rán /tū rán xià qǐ le dà yǔ.
It suddenly rained heavily outside.
The two words both can be used emphatically, at the beginning of a sentence.
tū rán /hū rán, tā de diàn huà xiǎng le.
Suddenly, his phone rang.
An obvious distinction between the two words is that tū rán (突然) also functions as an adjective which could be a predicate, an attributive or a complement. while hū rán (忽然) can only be an adverbial modifier in a sentence. Tū rán (突然) can’t be replaced by hū rán (忽然) in the following sentences.
duì wǒ lái shuō, zhè gè xiāo xī hěn tū rán.
The news is really sudden to me.
tā yī shí nán yǐ miàn duì jiā lǐ tū rán de biàn gù.
It was hard for him to face the sudden changes of his family.
zhè chǎng dà yǔ lái dé tài tū rán le.
The heavy rain was sudden.