The grammar books that I have seen state the Biblical Hebrew does not have past or future tenses, but rather perfect and imperfect (completed and not completed actions). Where does this idea come from? Examples of where it makes a difference? I have asked a number of Rabbis who said they were unfamiliar with this idea.

This is a huge topic, well beyond the scope of this concise framework! If you know dikduk, you should understand the Hebrew in the following article which clarifies many things, but also stresses that the laws of ancient Hebrew are less structured than modern languages. See: https://daat.ac.il/encyclopedia/value.asp?id1=2943