During the late Tang Dynasty, the renowned poet Zheng Gu showed remarkable talent from childhood, composing fresh and natural verses by the age of seven.
Once, the elder poet Sikong Tu visited Zheng Gu's father, Zheng Shi. Seeing that Zheng Gu was clever and endearing, Sikong Tu smiled and asked him, "Have you read my poems? Can you spot any flaws?"
Zheng Gu blinked his big eyes, tilted his head thoughtfully, and replied, "In your poem 'Evening View at Qujiang,' the lines 'At leisure I turn my head south of the village in the slanting sun, a pair of mandarin ducks alight on the ferry' carry profound meaning." Seeing how young Zheng Gu answered so appropriately, Sikong Tu could not help but pat his head with delight and said, "This child's future is boundless—he will surely become a renowned talent."
Zheng Gu was a poet of remarkable diligence, spending over a decade tirelessly composing verses through both bitter cold and scorching heat, amassing more than a thousand poems. After failing the imperial examinations twice in the capital, he was deeply disheartened. Instead of giving up, he traveled across the land, broadening his horizons, and his poetry grew ever more refined.
Around 885 AD, the poet Zheng Gu returned to Chang'an, determined to retake the imperial exams and win official rank. That winter, however, the Shatuo army invaded, forcing Emperor Xizong to flee to Fengxiang. Zheng Gu joined the refugees, but bandits ambushed them on the road and stole over a thousand of his poems. The loss struck him like a thunderbolt.
By the spring of 887 AD, the situation eased, and Zheng Gu returned to Chang'an, passed the imperial examinations, and was appointed county sheriff of Hu County (now Hu County, Shaanxi). He later served as Right Reminder and Director of the Bureau of Capital Officials, earning the nickname "Zheng the Director."
After entering officialdom, Zheng Gu discovered the court was rife with deceit and intrigue, and he soon grew weary of it. Before long, he resigned from his post and returned to his hometown, retreating to his Yangshan Study. There, he often composed poetry and prose with fellow scholars, intending to spend the rest of his days in peaceful seclusion.
A local monk named Qiji, who loved writing poetry and had gained some fame among local literati, had long admired Zheng Gu's reputation for talent. One day, he brought his poems to visit Zheng Gu, hoping for guidance.
Zheng Gu carefully read each of Qi Ji's poems. When he came across a poem titled "Early Plum Blossoms" with the lines "In deep snow before the village, several branches bloomed last night," he smiled and said, "If several branches have already bloomed, it's not truly early. Wouldn't it be more fitting to change 'several' to 'one'?"
After hearing this, Qi Ji's mind suddenly cleared. He felt Zheng Gu's suggestion was very reasonable and immediately bowed respectfully to express his sincere gratitude. When word spread, everyone called Zheng Gu Qi Ji's "One-Word Teacher."
Later, people used the idiom "Teacher of One Word" to refer to friends who correct a single character error and thereby deeply benefit others.
Source: *Supplement to the History of the Five Dynasties* by Tao Yue (Song Dynasty)
Meaning of the Idiom: Later, the Chinese idiom "一字之师" came to describe how a friend who corrects a single character error can deeply benefit others.