preposition
BAD : At night I keep my wallet behind my pillow.
GOOD : At night I keep my wallet under my pillow.
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behind = at the back of something: 'Lizzie ran and hid behind a tree.'
under (or beneath / underneath ) = directly below something, and sometimes touching or covered by it: 'Come and stand under my umbrella.' 'I eventually found the letter under a pile of old newspapers.'