Backshift is a technical term in the study of grammar. It may have been coined by Professor Quirk's team at UCL: it appears in their Quirk 1972, but not yet (2008) in the OED in this sense. (There is a separate meaning in the coal-mining industry.)
Backshift (or backshifting) is the phenomenon when the tense in which something is reported is further 'past' than its original, because of the fact that the sentence in which it appears is already in the past tense. This is most common in reported speech. The common cases in which backshift occurs are:
The present tense in actual speech, e.g. "I am happy", is reported as
The past tense in the actual speech, e.g. "He started the course here yesterday", is reported as
- He reported that he had finished his project.
The past perfect in the actual speech remains as past perfect, because that is the furthest back that the tense system of the English verb goes. It may be suggested by some adverbial: "She had finished her project" might become
- She said that she had already finished her project where the adverb already suggests a time 'before' the past perfect.
Future meaning in English is usually expressed with the inflection known as the present tense (the auxiliary verbs will amd shall are present in form). So the first rule above applies. The (future meaning) in the present tense in actual speech, e.g. "I will go to London tomorrow", is reported as
The original quotaion from Quirk 1972 is "If there is (semantically) a shift into the past in the reporting clause, there is a corresponding shift into the past (or if necessary, further into the past) in the reported clause." (p. 786)