I am reading a book about English verbs. The first chapter gives some basic definitions, and said that "be" can be used as a lexical verb and as an auxilliary verb. For example: "I am tall" or "I am running"; or "He is happy" or "He is reading".
My question has several parts:
1) Why can the "be" verb be used in these different ways?
2) Why does English require this "helping" verb for the progressive tenses. When you think about it, what is it really "helping?" It seems only useful in the sense of distinguishing the simple tenses from the progressives. Why not say "I now read" or "I now reading" and use "now" or some other word as the "helper?"
3) When you say "I am [word]," you are saying the subject has this quality, is in this state of being, or is the word. For example "I am tall," means you have this characteristic, "I am sad," means you are in this state of being, and "I am a fireman" means that you and fireman are the same. So, if you say "I am reading," it seems that you are somehow saying that you and the activity of reading (the gerund sense of the word) are the same. There seems to be a kind of relationship between you and "reading." Is this what's really going on but is just invisible to must of us? Is there some history to the use of "be" that addresses this (or is this more related to the development of "ing" in English?)