'There is no try'
One line from the Star Wars movies that I don't particularly like (okay, it bugs the heck out of me!) is Yoda's 'Do or do not. There is no try.' Can you imagine Elrond or Gandalf saying this to Frodo when he's commissioned to undertake the quest of Mount Doom - knowing that the destruction of the Ring is completely beyond his ability? If 'there is no try,' what's the point of sending him at all?
From Tolkien's writing, both fiction and nonfiction, we get the message that "try" does, indeed, exist. In fact, it can be very powerful. What's the basis for this difference between Yoda and Gandalf?
I think it goes back to the discussion of the Force in an earlier essay. The Force is an impersonal power that's "used" by a person either for good or evil. In learning to be a Jedi, Luke has to learn to use the Force, which really means to control it for his own purposes. You either use the Force or you don't: there is no try. Importantly, the ability to use the Force rests within the person using it. A Jedi using the Force is relying on his own ability - both innate and learned. It's no accident that Yoda comes across as a Zen master rather than a Christian teacher.
The Christian belief in a personal God puts the control in a completely different place. As is said in The Chronicles of Narnia, Aslan isn't a tame lion. Someone growing in Christian holiness isn't learning to "use" God, but to allow God to use them. The word "allow" is vital here; the person always has a choice. God isn't a Jedi using us in the way that Luke uses the Force. The Force has no will of its own; we do, and God has decided not to take that away from us. There are very real persons on each side of the relationship between God and us.
The Power behind try in Tolkien's writing is, ultimately, the power of God. Our calling, like Frodo's, is to carry the Ring as close to Mount Doom as we can, and to trust God to take it from there. Like Aragorn's, it's to prepare and hold ourselves in readiness for the way God wants to use us. Like Sam's, it's to go into the unknown purely out of love. None of these characters had control over the result of his calling - each was called to try.
Try is also a strong group concept. For one Jedi acting alone, Yoda's "There is no try" would make sense; one person's failure to do would result in complete failure. In a strong group cosmos such as Tolkien's, inadequate people trying together for the good of the community can do amazing things. This isn't just a practicality, like "Many hands make light work." Try is a spiritual force uniting people and recognizing that one of us - or even all of us - can't do it on our own.
Looking at things from Tolkien's point of view, I'd say that "Do or do not" is an invitation to despair. Denethor believed Gondor would either stand or fall on its own ability to defend itself, and he saw no hope for that - so gave up on try. To try, on the other hand, is to accept hope without assurance - to believe that there is a power beyond our control that can and will act when our try is not enough.