J.R.R. Tolkien based much of Quenya’s sound and structure on Finnish.
Quenya is one of Tolkien’s Elvish languages and is associated with the High Elves, especially the Ñoldor and Vanyar. Tolkien was a professional philologist, and his invented languages were designed with historical development, sound patterns, grammar, and vocabulary rather than as collections of isolated fantasy words.
Finnish particularly impressed Tolkien because of its richly inflected grammar and distinctive sound. He encountered the language while studying and described the discovery of the Finnish epic Kalevala as influential in his imaginative and linguistic development. Quenya is not simply Finnish with new vocabulary: Tolkien reshaped its grammar and words, and connected it to an internal history within his mythology.
Welsh also influenced Tolkien, especially the sound and development of Sindarin. Latin, Greek, Old English, and other languages contributed to his philological interests and to elements of his fictional languages and names. The common mix-up is therefore treating Finnish as the basis of all Elvish languages; it is chiefly linked to Quenya.