是…的
Ruleshi + emphasized element + verb + de: highlights the time, place, or manner of an action already known to have happened.
Curiosity
In "he came yesterday," how do you stress that it was specifically yesterday?
Intuition
shi...de spotlights the part you want to stress by sandwiching it between shi and de. The event is taken as known; the focus falls on "when, where, how."
Visualization
Wrap the plain [he][yesterday][came] into [he][shi][yesterday][came][de]. shi goes before the focus, de at the end.
plain statement (no emphasis)
他subject昨天time来verb
shi…de emphasis (stresses the time)
他subject是是 (cleft frame)昨天emphasized element来verb的的 (closes 是…的)
Essence
shi (often droppable) + emphasized element + verb + de. Use it only to stress added detail (time, place, manner, agent) of an action already realized; not for the future or unrealized events. The negative is bu shi...de.
Examples
他是昨天来的。
It was yesterday that he came.
我是坐飞机来的。
It was by plane that I came.
这本书是在北京买的。
It was in Beijing that I bought this book.
Mini-quiz
Which stresses "it was by train that he came"?