What is a verb tense shift?
It is a change of verbs' tenses between direct speech and indirect speech. See explanation.
A verb tense shift is a process of changing verb tenses when transferring between direct speech and indirect speech.
In direct speech you cite other people's words direct in quotation marks.
In indirect speech you say what other people say in other way.
Direct speech: Tom said "I am ill."
Indirect speech Tom said, that he was ill.
Direct speech: Kate said "I will marry Tom."
Indirect speech: Kate said, that she would marry Tom.
Direct speech: Mike said: "I was home at 8 p.m."
Indirect speech: Mike said, that he had been/was home at 8 p.m."
In future tense forms will changes to would, and shall to should.