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Put sugar in the egg and stir.
Literal
Egg [to-に] sugar [object-を] put, mix-[imperative].
Note the specific particle choice: 卵に (the eggs are the destination/recipient of the sugar) + 砂糖を (the sugar is the direct object). Reversing these would change the meaning. 入れる ('to put in') takes this に+を frame whenever adding one thing to another.