How do I say 'today / tomorrow / yesterday'?

今天, 明天, and 昨天 are the building blocks of Chinese time expression — they are each one word, not a preposition + noun like English “to-day” or “yester-day.” The pattern is transparent: 今 (present) + 天 (day), 明 (bright/next) + 天, 昨 (previous) + 天.

The most important grammar rule: time words in Chinese come before the verb, typically right after the subject. 我今天开会 (I today have meeting), not 我开会今天. This subject-time-verb order is one of the most reliable patterns in Chinese and getting it right removes a major source of translated-sounding speech.

Chinese does not need prepositions with time. “On Tuesday” is just 星期二; “in the morning” is just 早上. Adding 在 before every time expression is a common English-speaker habit that makes sentences wordier than they need to be.