"Ours is not to reason why. Ours is but to do and die", by Alfred Lord Tennyson. I am a non-English speaker and have a hard time understanding the grammatic logic of this phrase. Please explain?

"Ours" here actually means "our duty." Tennyson was saying the horsemen had no choice about obeying or even thinking about the command, but to blindly obey it even though they knew it would cost them their lives.

Another similar poetic expression is, for example, "It was not mine to choose," meaning, "I didn't have the privilege of choosing," or, "It wasn't his to say," meaning, "He didn't have the option of saying." Usually we will have omitted the words duty, right, or privilege in this construction.

Best wishes with your studies--the one who asks learns.

This fairly well says it all:

Direct from "The Charge of the Light Brigade."

'Forward, the Light Brigade!'
Was there a man dismay'd ?
Not tho' the soldier knew
Some one had blunder'd:
Theirs not to make reply,
Theirs not to reason why,
Theirs but to do & die,
Into the valley of Death
Rode the six hundred.

- See more at:

http://www.nationalcenter.org/ChargeoftheLightBrigade.html

A translation into simpler prose would be 'Our duty is merely to do what we are told to do even though we die as a result'. English is very good for re-arranging the order of words, and poetry is especially made up of re-arranged words. Poets put the most meaning into the fewest words (except perhaps for the 'poets' of the Poets' Corner' of the local newspaper William MacGonnegal whose name I can never spell).