Backshift in Reported Speech

direct speech	reported speech
He said: "I feel sad."	He said that he felt sad.

In simple terms, the structure of reported speech is:

reporting clause [+ conjuntion] + reported clause

reporting clause	conjunction	reported clause
John said	(that)	he was hungry.
John's original words: "I am hungry."

We sometimes change the tense of the reported clause by moving it back one tense. For example, present simple goes back one tense to past simple. We call this change "backshift".

When do we use backshift?

We use backshift when it is logical to use backshift. So, for example, if two minutes ago John said "I am hungry" and I am now telling his sister, I might NOT use backshift (because John is still hungry):

• John just said that he is hungry.

But if yesterday John said "I am hungry" and I am now telling his sister, I would likely use backshift:

• Yesterday, John said that he was hungry.
  
  
  [We hope that John has eaten since yesterday ;-) ]

So we use backshift SOMETIMES but not always. And WHEN we use backshift, here's how it works with these common tenses and modals:

backshift
this	goes back to this
present simple	→	past simple
present continuous	→	past continuous
past simple	→	past perfect
present perfect
past continuous	→	past perfect continuous
can	→	could
may		might
will		would
shall		should

We NEVER use backshift when the original words are:

• past perfect
• could
• might
• would
• should

Remember:

• If a situation is still true, backshift is optional.
• For a general truth there is no need for backshift.

Look at the following examples. See if you can understand when and why they use backshift:

tenses and modals	direct speech	reported speech
present simple*	He said, "I like coffee."	He said (that) he likes coffee.
		He said (that) he liked coffee.
present continuous*	She said, "Moo is living here with us."	She said Moo is living there with them.
		She said Moo was living there with them.
past simple	John said, "We bought a house last week."	John said they had bought a house the week before.
present perfect	Ram said, "I haven't seen Avatar."	Ram said he hadn't seen Avatar.
past continuous	Wayne said, "Were you watching TV when I called."	Wayne asked if I had been watching TV when he called.
past perfect**	Ati said, "I had never lived in Thailand before."	Ati told us that he had never lived in Thailand before.
can	She said, "Tara can't swim."	She said Tara couldn't swim.
		She said Tara can't swim.
could**	He said, "Could you swim when you were three?"	He asked me if I could swim when I was three.
may	She said: "I may be late."	She said she might be late. (and she was late)
		She said she may be late. (the time to be late has not yet arrived)
might**	She said, "I might come early."	She said she might come early.
will	She said, "I'll call you tomorrow."	She said she would call me the next day.
		She said she will call me tomorrow. (tomorrow has not come)
would**	She said, "I wouldn't like to go."	She said she would not like to go.
shall	He said: "Shall I open the door?"	He asked if he should open the door.
should**	John said, "You should come here."	John said I should go there.
must	The kidnapper phoned me and said: "You must come here now."	The kidnapper phoned me and said I had to go there then.
	Ati said, "I must find a job next year."	Ati said he must find a job next year. (next year hasn't come yet)
have to	Tara said: "I have to do my homework."	Tara said she had to do her homework.
		Tara says she has to do her homework.

* if still true, change is optional (sometimes a matter of emphasis)

** never changes

https://www.englishclub.com/grammar/sentence/reported-speech-backshift.htm