The adverb 忽而 "suddenly"
The adverb 忽而 
indicates that an action was unexpected.
indicates that an action was unexpected. Pay attention
忽而 is usually used in written language.
Scheme
subject
忽而
predicate
Examples
他走着走着，忽而停住了
He was walking and then he suddenly stopped
tā zǒuzhe zǒuzhe, hū'ér tíngzhù le
我忽而听见有人大叫一声，不知出了什么事
I suddenly heard someone screaming. I don't know what happened
wǒ hū'ér tīngjiàn yǒurén dàjiào yì shēng, bù zhī chū le shénme shì
忽而传来了枪声
Suddenly a shot rang out
hū'ér chuánlái le qiāngshēng
The structure 忽而…忽而…
忽而 can also be used as a part of the structure 忽而…忽而…
This structure indicates alternating actions or situations.
In the structure, both ellipses are replaced with predicates, which can be expressed by verbs, verb phrases, or adjectives.
This structure indicates alternating actions or situations.
In the structure, both ellipses are replaced with predicates, which can be expressed by verbs, verb phrases, or adjectives.Scheme
subject
忽而
predicate 1
忽而
predicate 2
Examples
我忽而充满希望，忽而担心
Sometimes I'm hopeful, sometimes I'm worried
wǒ hū'ér chōngmǎn xīwàng, hū'ér dānxīn
天气忽而冷忽而热
Now it's cold, before it was hot
tiānqì hū'ér lěng hū'ér rè
雨忽而下，忽而停
One moment it starts raining and the next moment it stops raining
yǔ hū'ér xià, hū'ér tíng