[jallanite]

Originally Posted by TheLostPilgrim

Maiar are Ainur, correct?[

Yes.

I've never understood the distinction between the Valar and the Maiar--I thought they were all the same sort of "being." What sets the Valar from the Maiar?

From the “Valaquenta” in the published Silmarillion, the beginning of the section “Of the Valar”:

The Great among the spirits [the Ainur] the Elves name the Valar, the Powers of Arda, and Men have often called them gods. The Lords of the Valar are seven; and the Valier, the Queens of the Valar are seven also.

The names of the fourteen Valar follow. Both Valar and Maiar are Ainur and of the same kind. But the Valar are significantly more powerful than the Ainur and have more authority. Melkor was originally more powerful than any of the Valar.

Also, were the Maiar present at the Ainulindalë, were all the Ainur created at the same time?

No. It is mentioned in the “Valaquenta” that the Valar Mandos (more rightly called Námo) and Lórien (more rightly called Irmo) were brethern and Mandos was the elder of the two and Lórien was the younger.

Did the Maiar help shape Arda? Or were the Maiar created later?

Presumably the Ainur helped shape Arda. There is no indication that the Maiar were created after the Valar. At least the Vala Tulkas entered Arda some time after the other Valar and it is possible also that the Maiar also came into Arda at different times.

1) Would Gandalf, Saruman, Radagast, etc--Those Maiar who were sent to Middle Earth--have any memories of their prior selves, of the Beginning? Of themselves before they were "Wizards"? Would they still retain the memory or knowledge of their true selves, the extent of what they truly were, and where they came from? Would Saruman, by the time of his Fall, have forgotten that he was a Maiar, who shaped the history of Middle Earth in a small way, that he was ultimately a servant or creation of Eru?

Mostly unknown. Tolkien indicates in his article “The Istari” in Unfinished Tales:

For it is said indeed that being embodied the Istari had need to learn much anew by slow experience, and though they knew whence they came the memory of the Blessed Realm was to them a vision from afar off, for which (so long as they remained true to their mission) they yearned exceedingly).

2) Would Gandalf still have been Gandalf? What I mean is, once he returned to the West, would he still have retained his personality, emotions and his memories of Middle Earth, of Frodo, etc? Would his spirit, his nature, still be recognizable as "Gandalf" in his natural form as Olorin? I don't mean who Gandalf was physically; I mean who he was in terms of his personality, his traits, etc?

Presumably so, for the most part. In the Two Towers when the resurrected Gandalf meets again Strider, Legloas, and Gimli, Gandalf for a short time appears preoccupied and to not fully recall his former life in Middle-earth, but this vagueness soon departs from his mind.

3) Have we ever gotten any indication of what the Istari, or Maiar in general, look like in their "true" forms? I've read the Istari were simply "clothed" in the bodies of old men...I wonder what their true forms looked like.

In their innate forms the Ainur were unclothed spirits and thus presumably invisible and intangible. Tolkien only mentions in the “Valaquenta”:

But fair and noble as were the forms in which they [the Valar and the Valier] were manifest to the Children of Ilúvatar, they were but a veil upon their beauty and their power.

This implies that in their unveiled power the Valar were not manifest to the Children of Ilúvatar.

4) Will all the Ainur--including Gandalf--exist until "The End"?

Presumably most of them.

5) Of the Ainur in general--the Ainur are said to be the offspring of Eru's thought; Essentially, almost sort of parts of Eru. Would Eru's power or force or whatever you'd want to call it be reduced when say, the spirits of Morgoth or Sauron or Saruman are extinguished? Would those parts of Eru simply cease to exist, or is Eru unchanging and would retain the fullness of his spirit?

By common Christian theology Eru would be reduced by nothing he created.

Getting a bit religious, but, since the Ainur were the pure offspring of Eru's thought, could it be that the actions of some of his Ainur (Sauron, Saruman) were ultimately part of Eru's "Grand Plan" for Middle Earth? Ultimately, what he intended, that in doing what they did, they played a part in Eru's theme--without even consciously knowing they were doing so?

That is Tolkien’s intended thought.