Hundreds of times I have sat for hours in front of texts the meaning of which I understood perfectly, and yet been unable to see how they ought to be put into English in such a way as to re-embody not merely a series of correct dictionary meanings, but also the emphasis, the tone, the eloquence of the original.

Arthur Waley (1889–1966) English orientalist, sinologist, literary translator

“Notes on Translation,” Atlantic Monthly (Nov 1948)
    (Source)

Reprinted in The Secret History of the Mongols (1963).

A variation of this quote is frequently found which appears to have been synthesized by Simon Leys (attributed to Waley) in his 2008 essay, "The Experience of Literary Translation":

Hundreds of times I have sat, for hours on end, before passages whose meaning I understood perfectly, without seeing how to render them into English.