These seem like holdovers from PIE please explain the roots of these functions and how they operate a little bit more clearly. My latin teacher said the passive pariphrastic could equal must, however I don’t see it and I would like a little help. I would be greatly apreciated if someone would trace the evolution of these concepts and how they are most commonly used in latin.
It isn’t obvious from the grammar/syntax that the passive periphrastic means “must.” That’s why it is “periphrastic.” However, it almost always means something that must or ought be done–just as in the famous phrase “Carthago delenda est.” It’s simply idiomatic.
Because the passive periphrastic is passive, the person who is doing the action cannot be the subject. The dative of agency is what is used instead. The ablative of agenct (ab+agent) is sometimes used, usually to clarify or emphasize the agency. I’m not sure exactly why it isn’t used more often. I’ve heard the dative of agency described as a special use of the dative of possession (the necessity belongs to the agent) or a development of the dative of reference (referencing the general phrase to the agent).
Ablatives usually modify or limit ideas–and that’s just what they do in a sentence with the ablative absolute. It describes the circumstances around the sentence. I know it seems strange because it is particularly foreign to an English speaker, but it makes sense once you get the hang of it. It is an efficient way to handle subordinate clauses. It’s absolute because it is “free” (absolutus) from the rest of the sentence–i.e. it refers only to attending circumstances and not to nouns mentioned in the main clause.
As I currently undersand it the Ablative has a few major uses that vary depending on the structure of the sentence.
It denotes
In
With
By
From
About
within which
and others.
All of these uses are related, they all hark back to a common root. It doesn’t really matter that the ablative of means signifies by. What is important is understanding the uses of the ablative case itself. That is among other things what I am asking because understanding the roots of the ablative simplify the rest of the equation. Hence the asking about PIE again.
Discipulum Sum et supra grammaticos
Just kidding.
PIE actually had more cases than Latin. The Latin ablative is actually a conflation of THREE PIE cases:
-the “true” ablative, which is an ablative of separation (which is why the word ablative comes from ablatus, meaning carried away)
-the locative, which signified place where (and by analogy time when), and
-the instrumental, which signified manner/means/circumstance.
I’m not sure exactly what the theory is on when/how/why this happened. I’m sure you can learn how when you go to graduate school. 
PIE actually had more cases than Latin. The Latin ablative is actually a conflation of THREE PIE cases:
-the “true” ablative, which is an ablative of separation (which is why the word ablative comes from ablatus, meaning carried away)
-the locative, which signified place where (and by analogy time when), and
-the instrumental, which signified manner/means/circumstance.
I know that someone else was happy enough to fill me in on that when I asked about the similarity between latin and greek ablative vs dative uses. Just would like to know why so many grammatical terms with the ablative when if you understand it’s essential functions the rest fall into place. (my english grammer she’s not so good eh. to use a quote from my grandfather.)