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.