Backshift

Backshifting (/bækʃɪftɪŋ(g)/) or backshift (/bækʃɪft/) occurs in indirect reported speech by changing verbs in the present tense to the aspect-corresponding past tense following a preterite of a reporting verb.

For example instead of:

• “Dave said he is not going out tonight because he has got a lot of work to do and wants to finish it.”

We usually say:

• “Dave said he was not going out tonight because he had got a lot of work to do and wanted to finish it.”

Form

Sometimes backshift is required; at other times it is optional.

Optional backshift

Optional backshift occurs in things that are general truths, i.e. general time. For example:

• Alice to Bob: “There are eight planets in the Solar System.”
• Bob to Charlie: (without backshift): Alice said there are eight planets in the Solar System (or with backshift) Alice said there were eight planets in the Solar System.

Mandatory backshift

Backshift should occur when something is in past finish time, though it wasn’t when originally stated. For example:

• Alice to Bob: “I’m eating dinner now.”
• Bob to Charlie: "Alice said she was eating dinner then." but not *"Alice said she is eating dinner then.”

Application

This applies to all finite verbs, including auxiliary verbs, and independent lexical verbs. In turn, the auxiliary verbs include full modal verbs, e.g. can -> could, will -> would, etc.

Miscellaneous points

This is one instance when the past tense can be used to talk about future time.