Yesterday's tense is the PAST tense, which is used to refer to events that have already happened. It is used to describe actions, states, or events that occurred in the PAST. The PAST tense is typically formed by adding -ed to the end of a verb, such as 'walked' or 'jumped'. It can also be formed with irregular verbs, such as 'went' or 'SAW'. The PAST tense is an important part of language and is used to communicate information about events that have already taken place.