'I will take the Ring, though I do not know the way.'
These are perhaps some of the best known words said by Frodo in all of The Lord of the Rings, often quoted and commented upon. 'I shall', however, is the normal way to express the future tense in the first person singular. Before commenting upon the choice Tolkien made here in preferring 'will' to 'shall', it will be useful to examine the times character say 'I shall' and 'I will' throughout the discussion in The Council of Elrond. Let's start with 'I shall'. It is the default, and there are only three instances.
(a) And now that part of the tale that I shall tell is drawn to its close. (FR 2.ii.245)
(b) It was hot when I first took it, hot as a glede, and my hand was scorched, so that I doubt if ever again I shall be free of the pain of it. (252, emphasis original, indicating quotation of a written document)
(c) I had thought of putting: and he lived happily ever afterwards to the end of his days. It is a good ending, and none the worse for having been used before. Now I shall have to alter that: it does not look like coming true.... (269)
The speakers here are, in order, Elrond, Isildur (as quoted by Gandalf), and Bilbo, three very different characters. Each use of 'shall' here indicates nothing more or less than the speaker's opinion of what is or is not going to happen. There is little to say or argue about here so far.
Turning to 'I will', we find nineteen instances uttered by nine speakers: Elrond, Isildur, Aragorn, Bilbo, Gandalf, Radagast, Boromir, Gwaihir, and Frodo.
(A) 'And I will begin that tale, though others shall end it.' (Elrond, 242)
(B) '"This I will have as weregild for my father, and my brother," (Isildur, 243)
(C) 'And this I will say to you, Boromir, ere I end.' (Aragorn, 248)
(D) 'But now the world is changing once again. A new hour comes. Isildur's Bane is found. Battle is at hand. The Sword shall be reforged. I will come to Minas Tirith.' (Aragorn, 248)
(E) 'Very well,' said Bilbo. 'I will do as you bid. But I will now tell the true story, and if some here have heard me tell it otherwise' – he looked sidelong at Glóin – 'I ask them to forget it and forgive me.' (Bilbo, 249)
(F) 'But for my part I will risk no hurt to this thing: of all the works of Sauron the only fair. It is precious to me, though I buy it with great pain. (Isildur, 253)
(G) 'And now I will answer Galdor's other questions. What of Saruman? (Gandalf, 256)
(H) '"I will go to Saruman," I said. (Gandalf, 257)
(I) '"I will do that," he said....' (Radagast, 257)
(J) "Well, the choices are, it seems, to submit to Sauron, or to yourself. I will take neither. Have you others to offer?" (Gandalf 260)
(K) '"Then I will bear you to Edoras, where the Lord of Rohan sits in his halls," he said; "for that is not very far off." (Gwaihir, 261)
(L) 'Nor is it now, I will swear,' said Boromir. 'It is a lie that comes from the Enemy.' (Boromir, 262)
(M) "If this delay was his fault, I will melt all the butter in him. I will roast the old fool over a slow fire." (Gandalf, 263)
(N) 'I fear to take the Ring to hide it. I will not take the Ring to wield it.'
'Nor I,' said Gandalf. (Elrond, followed by Gandalf, 267)
(O) 'I will take the Ring,' he said, 'though I do not know the way.' (Frodo, 270)
(P) 'But if you take it freely, I will say that your choice is right....' (Elrond, 270)
In contrast to the three instances of 'I shall', 'I will' quite clearly communicates intent, desire, or choice (whether acceptance or refusal). Particularly interesting is that Isildur twice uses 'I will' (B, F) of what he intends to do or not do in connection with the Ring, in contrast with his use of 'I shall' (b) to denote what he expects will be the case with the pain the Ring has caused him. Mark also Elrond's explicit and Gandalf's implicit use of 'I will' to indicate their refusal of the Ring (N). Elrond makes clear (P) that his approval of Frodo's choice or intention is conditional (O). Elrond, moreover, has previously expressed an opinion about the wisdom of 'taking' the Ring:
'Isildur took it! That is tidings indeed.' [said Boromir]
'Alas! yes,' said Elrond. 'Isildur took it, as should not have been.' (243)
On this showing, Frodo's 'I will take the Ring' occupies a much greyer area than it seems to do at first glance. His courage and his humility are still there, just as they always have been, but the ambiguity and the peril of 'I will' are also in keeping with the desire he had felt only the night before to strike Bilbo when he reached out for the Ring which Frodo was quite reluctant to show him (231).
I hope to study these uses of 'I shall' and 'I will' further in a later post, which will also explore the distinction more widely in The Lord of the Rings.