矣: the classical sentence-final particle
【高58】助词 / particles
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
矣 closes a sentence in Classical Chinese, or in writing that imitates it, and marks a state that has become real, or a change, much as 了 does in speech, with a graver or more exclamatory tone. In 《师说》 Han Yu writes: 师道之不传也久矣 (“it has been a long time since the way of the teacher was handed down”).
也 or 矣? Classical Chinese has two very common final particles. 也 states a judgement or a plain fact (师者，所以传道受业解惑也: “a teacher is there to…”). 矣 reports that something is now so, or has become so (久矣, 难矣, 远矣). It follows an adjective or a stative verb, sometimes with 已: 已……矣 (“already…”).
Today 矣 is no longer used in ordinary sentences: you say 了 (hsk1-g38, hsk1-g46). It survives in quotations, in set phrases (悔之晚矣, 足矣, 往事已矣) and in writing that imitates the classical style. It always comes at the end of a sentence, and never together with 了.
Examples
人生得一知己足矣。
Rénshēng dé yì zhījǐ zú yǐ.To find one true friend in life is enough.
事已至此，悔之晚矣。
Shì yǐ zhì cǐ, huǐ zhī wǎn yǐ.It has come to this; regret is too late.
往事已矣，何必再提？
Wǎngshì yǐ yǐ, hébì zài tí?What is past is past; why bring it up again?
师道之不传也久矣！
Shīdào zhī bù chuán yě jiǔ yǐ!It has been a long time since the way of the teacher was handed down! (Han Yu, 《师说》)
欲人之无惑也难矣！
Yù rén zhī wú huò yě nán yǐ!To want people to be free of doubt is hard indeed! (Han Yu, 《师说》)
古之圣人，其出人也远矣。
Gǔ zhī shèngrén, qí chū rén yě yuǎn yǐ.The sages of old stood far above ordinary people. (Han Yu, 《师说》)
Common mistakes
✗ 我吃饭矣。 → ✓ 我吃饭了。 (in a modern sentence, 矣 only parodies the classical style).
✗ 足矣的人生 → ✓ 人生得一知己足矣。 (矣 closes its sentence; it never modifies a noun).
✗ 他已经走矣了。 → ✓ 他已经走了。 (矣 and 了 do the same job, so you use only one).