The emergence of the new copula 是 shì is one of the important phenomena of post-classical language.
The copula 是 shì originated from the demonstrative pronoun 是 shì. Gradually, 是 shì is transformed into a copula and replaces the former 也 yě.
We find the first signs of this already in the late classical language, but this usage of the word only becomes the norm in the post-classical period:
汝是阿誰 rǔ shì āshuí "Who are you?"
我是汝妻 wǒ shì rǔ qī "I am your wife."
But in the early post-classical period, 是 shì is also found in its previous meaning:
知是故人來 zhī shì gù rén lái "She realized: it was her ex-husband who had arrived."
非 fēi is still used as a copula in a negative sentence at this time. The combination 不是 bù shì appears much later.
Wedding preparations in ancient China were always preceded by finding out whether the upcoming marriage would be happy.
To do this, it was necessary to know the birth dates of the bride and groom, so that a horoscope could determine their compatibility.
The wedding day was determined with equal care. The chosen date was considered most "auspicious" if the six cyclic signs denoting the year, month, and day of the celebration "corresponded" to each other (for example, year 甲子 jiǎ zǐ, month 乙丑 yǐ chǒu, day 丙寅 bǐng yín). Sometimes, the wait for the right day was very long, several months. Therefore, the groom's joy at learning that the auspicious day would arrive in the same week is understandable.
Unlike modern language, for which the transcriptional adaptation of borrowings is generally uncharacteristic, in ancient times this method of transmitting them played a leading role. It's only natural that borrowing by sound generally involved some distortion of the phonetic appearance of the words being adapted.
A striking example of this is the ancient Chinese word 琉璃 liú lí "enamel" (from the Sanskrit veluria). Note that glass, borrowed from the West, was designated by the same word in the first centuries AD.