是 + Detail + Verb + 的
The 是…的 (shì…de) construction is used to emphasize or confirm specific details about an action that has already occurred, such as the time, place, manner, or agent. The 是 can sometimes be omitted, but 的 at the end is essential. For example, 他是昨天来的 emphasizes that he came yesterday.
这本书是在北京买的。
zhè běn shū shì zài Běijīng mǎi de.
This book was bought in Beijing.
我们是坐高铁去上海的。
wǒmen shì zuò gāotiě qù Shànghǎi de.
We took the high-speed rail to Shanghai.
她是去年毕业的,现在已经工作一年了。
tā shì qùnián bìyè de, xiànzài yǐjīng gōngzuò yī nián le.
She graduated last year and has already been working for a year now.