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Don't count your chickens before they hatch.
Literal
Egg [subject-が] hatch before chick [possessive-の] number [object-を] count-don't.
A direct translation of the English proverb (literally 'before the eggs hatch, don't count the number of chicks'). かえる in this context is the verb 孵る ('to hatch'), not 帰る ('to go home') or 変える ('to change') — rendered in kana because the kanji is uncommon. The negative imperative ~な suits the proverb register.