"I am doing my homework" She said that she was doing her homework.
"I did my homework" She said that she has done her homework.
"I have done my homework" She said that she had done her homework.
yesterday->that day, the previous day; tomorrow-the next/following day; a week ago-a week before и т.д.
представь себе ступеньку из времен английского языка. каждый раз, когда превращаешь прямую речь в косвенную, возвращайся на предыдущую ступеньку.
надеюсь, все понятно!