The terms describes the function of two or more clauses in a sentence in which one is conditional upon another ("If this . . . then that" or "That when this"). The protasis is the conditional clause (the "if" or "when" statement) while the apodosis is the consequential, or principal, clause (the "then" statement).
In the sentence, e.g., "If you give me your promise not to harm anybody I will return your axe to you," the clause "I will return your axe to you" is the apodosis, while the clause "if your give me . . . . anybody" is the protasis. Or in the sentence "She started screaming when the dog came towards her," "when the dog came towards her" is the protasis and "she started screaming" the apodosis.
Both terms are derived from Greek: apodosis from apo 'back' and didonai 'to give,' and protasis from pro 'before' and tasis 'stretching.'
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