In a school paper, my son wrote the sentence, "In 1763, the stalemate would be broken." His teacher told him to avoid the "past progressive tense." The phrase "would be" is clearly not an example of the past progressive tense--but I can't quite figure out what tense it is. Can anyone help?
In your sentence, would is a modal verb that doesn't have a tense per se.
From the Cambridge dictionary:
Modal verbs do not change in form to make different tenses.
All of the modal verbs can refer to present and future time. Only some of them can refer to past time.
In the case of your sentence, would is referring to a past event.
First, the sentence of your son is
In 1763, the stalemate would be broken.
so the verb is would be broken, and not just would be. It's a passive construction. The active counterpart would be:
In 1763, [something] would break the stalemate.
The use of would indicates a conditional mood. In this specific example, the tense is simple conditional (I would do smth).
N.B: Of course, here conditional doesn't imply there is a condition. It refers to a "future in the past" (i.e. future happenings with the past as a time reference).