The Chinese word "这天" is formed through a straightforward grammatical combination of the demonstrative pronoun 这 (zhè, meaning "this") and the noun 天 (tiān, meaning "day"). It functions as a demonstrative phrase meaning "this day" or "on this day". Unlike a single fused compound word, it operates more as a common collocation where the specifier "这" points to a specific, often previously mentioned or contextually understood day, thereby giving the general noun "天" a definite and particular reference. It is frequently used at the beginning of a sentence in narrative contexts to pinpoint and emphasize a specific date or time period being discussed.