昨天

day (日) + [sound: 乍] = yesterday (昨) · overhead sky (天) = a day gone by

昨 pairs the sun radical for 'day' with 乍, which mainly provides the sound zuó. 天 pictures a person (大) with a horizontal stroke above their head — the sky, the vast expanse overhead — giving us 'day' in the sense of a whole sky-span of time.

昨天我吃了一碗面条。

昨天下午他去图书馆看书了。