Number + 世纪
世纪 (shìjì) means 'century' and is used to refer to a hundred-year period. It is typically preceded by a number or an ordinal to specify which century is being discussed. For example, 二十一世纪 means 'the 21st century.'
二十一世纪的科技发展速度让人惊叹。
Èrshíyī shìjì de kējì fāzhǎn sùdù ràng rén jīngtàn.
The speed of technological development in the twenty-first century is astonishing.
那座教堂建于十八世纪,保存得很完好。
Nà zuò jiàotáng jiàn yú shíbā shìjì, bǎocún de hěn wánhǎo.
That church was built in the eighteenth century and is very well preserved.
很多历史学家认为二十世纪是变化最大的世纪。
Hěn duō lìshǐ xuéjiā rènwéi èrshí shìjì shì biànhuà zuì dà de shìjì.
Many historians consider the twentieth century to be the century of greatest change.