I'm going to take a prophetic leap here and predict this post will not become one of our most popular grammar posts, but grammar is not intended to be a popularity contest. Rather, it's about figuring out how to correctly master language. So let's dig in.
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Prophecy vs. Prophesy
Prophecy is a noun that means prediction. Specifically, it's a prediction that is delivered by a prophet, who is usually, though not always, a person instilled with their prognostic powers by a divine being. Prophecy tends to carry more weight than an everyday prediction, forecast, or educated guess.
Prophesy, on the other hand, is a verb that describes the action of making a prophetic prediction. As with the noun, a prophet most commonly prophesies under divine influence.
Here are a few examples:
Correct: The king often worried over the prophecy about his kingdom falling into darkness.
Incorrect: The king often worried over the prophesy about his kingdom falling into darkness.
Correct: She prophesied, "The divine goddess will bless this marriage with enduring happiness."
Incorrect: She prophecied, "The divine goddess will bless this marriage with enduring happiness."
As such, prophets can prophesy a prophecy, but they can't prophecy a prophesy.