Cleft sentences are a sentence type that can focus on a certain part of the sentence motivated by communication needs in a given discourse. Usually the verb 是 (sh`ı/ to be) is placed right in front of the constituent to be emphasized.
他是昨天買菜的 (t¯a sh`ı zu´oti¯an m˘ai c`ai de/ It was yesterday that he bought (some) food.)
→ "Yesterday" is emphasized.
他昨天是買了菜 (t¯a zu´oti¯an sh`ı m˘ai -le c`ai/ What he did was to buy food yesterday.)
→ The verb phrase "To buy food" is emphasized.
他昨天是在山景城買菜的 (t¯a zu´oti¯an sh`ı z`ai sh¯anj˘ıngch´eng m˘ai c`ai de/ It was in Mountain View that he bought food yesterday.)
→ The prepositional phrase "in Mountain View"
It is worth mentioning that DE (的) at the end of the sentence is frequently omitted, leaving behind sh`ı (是) as the most conspicuous focus marker.
To emphasize the undergoer that follows the verb, it must be separated from the verb and placed after the verb 是.
For instance, in the sentence 他昨天買的是菜 (t¯a zu´oti¯an m˘ai de sh`ı c`ai/ It was food that he bought yesterday.), two equivalent noun phrases 昨天買的 (zu´oti¯an m˘ai de) and 菜 (c`ai) are linked by the verb 是.
Reference: Chinese: a linguistic introduction by Chao Fen Sun