O Reported Speech (discurso indireto) transmite o que alguém disse sem citar as suas palavras exatas. Os tempos verbais fazem um backshift (recuam um passo no passado).
Direct: 'I live in London.' → Reported: She said she lived in London.
Direct: 'I am tired.' → Reported: He said he was tired.
Direct: 'I will call you.' → Reported: She said she would call me.
Quando o verbo introdutório é no passado (said, told, asked...), os tempos verbais recuam.
Present Simple → Past Simple: 'I work here.' → He said he worked there.
Present Continuous → Past Continuous: 'I am waiting.' → She said she was waiting.
Past Simple → Past Perfect: 'I saw it.' → He said he had seen it.
Will → Would: 'I will help.' → She said she would help.
Can → Could: 'I can swim.' → He said he could swim.
Outros elementos também mudam: pronomes, demonstrativos e expressões de tempo.
'I like this.' → She said she liked that.
'I'll do it today.' → He said he'd do it that day.
'Come here.' → She told him to go there.
'I saw him yesterday.' → She said she had seen him the day before.
As armadilhas clássicas para falantes de português
Estes são os erros mais frequentes dos falantes de português neste ponto.