How the verb steps back when you pass on someone's words
What happens to tenses, pronouns and time expressions when you pass on yesterday's words today.
She said she was tired and would call me the next day.
After a past reporting verb, the tense inside the reported clause moves one step back: a present form becomes past, a simple past becomes the past perfect, and the future auxiliary changes to its past-time form. This happens because the whole report is now anchored to a past moment of speaking. Pronouns adjust to the reporter's point of view, and time expressions tied to the original moment shift to their more distant equivalents.
Two things resist the shift: past modal forms have no further step back and stay as they are, and a reported fact that is still true now may keep its original tense. The commonest real-world error, though, is shifting the verb but forgetting to move the pronouns and time expressions with it, so the report contradicts itself.