是 + Emphasized Element + 的
The 是…的 (shì…de) construction is used to emphasize a specific detail about a past action, such as when, where, how, or by whom it was done. The emphasized element is placed between 是 and 的, drawing the listener's attention to that particular aspect.
我是去年来北京的。
wǒ shì qùnián lái běijīng de.
I came to Beijing last year.
这本书是他在图书馆借的。
zhè běn shū shì tā zài túshūguǎn jiè de.
This book is one he borrowed from the library.
她是坐地铁去上班的。
tā shì zuò dìtiě qù shàngbān de.
She takes the subway to work.