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Try and train yourself out of the habit of adding a single entry to the start of a list by using ++ and instead cons it on the front with :.

E.g., write

every n (x:xs) = x : every n (drop (n-1) xs)
-- Rather than
every n (x:xs) [x] ++ every n (drop (n-1) xs)

(:) is O(1) because you're just adding a node to the head of a linked list, whereas ++ is O(n) in the length of the first list because Haskell has to walk to the end of the first list before appending the second one.

Although it doesn't really matter here because the first list is only length one, it's an antipattern one tries to avoid. e.g. compare the runtime performance of

rev :: [a] -> [a]
rev (x:xs) = rev xs ++ [x]
rev [] = []
-- with

rev' :: [a] -> [a]
rev' xs = revH xs []
  where
    revH (x:xs) ys = revH xs (x:ys)
    revH [] ys = ys

(Getting ready to teach intro Haskell again...)