Example sentences
zuó昨tiān天nǐ你qù去nǎ哪ér儿le了？
Where did you go yesterday?
zuó昨tiān天hěn很lěng冷。
It was cold yesterday.
About this word
昨 means yesterday and 天 is day, so 昨天 is literally "yesterday day". 昨 cannot stand on its own in modern speech; it lives inside 昨天, 昨日, and 昨晚 (last night). The day words form a tidy set: 前天 (the day before yesterday), 昨天, 今天, 明天, 后天 (the day after tomorrow). Time words like this sit before the verb and need no preposition: 我昨天去了商店, not 我在昨天去.
The story behind the character
昨
日 (time) plus 乍 (sound). The 日 marks it as a time word, the 乍 gives the sound. It means the day before.
天
大 is a person with arms spread. Add one stroke on top and you get 天. The space above a person's head. So it didn't start as "sky," it started as "the top of your head." Only later did it climb up to mean heaven.