The “shì... de” construction is commonly used in Chinese to emphasize “when”, “where”, “how” an event has happened.
e.g.
Shì shénme shíhou diū de? = When was it that you lost it?
Shì jīntiān zǎoshɑnɡ diū de. = It was this morning that I lost it.
Tag:
Mandarin,
Putonghua,
HSK