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From grammarly

In most cases, the subjunctive form of a verb is usually the third-person form of the verb with the ‑s dropped, but the verb to be is a special case. The subjunctive is used after certain expressions that contain an order or a request, a hypothetical, or a wish.

The phrase "a hypothetical" being used as a noun phrase there.

According to Cambridge Dictionary, "hypothetical" is an adjective and the noun form of it is "hypothesis".

Is that a typo or some kind of grammar rule?

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Yes, "hypothetical" is being used as a noun, to mean "something that is hypothetical". It doesn't mean "hypothesis" in that sentence; that's why they did not use the word "hypothesis". Something can be hypothetical without being a hypothesis, just like something can be theoretical without being a theory. I think in this case it's a specific term being used to describe the grammar.

Merriam-Webster lists hypothetical as possibly being a noun, but doesn't define it. Wikipedia defines hypotheticals as:

possible situations, statements or questions about something imaginary rather than something real

Using an adjective as a noun happens often enough, but you can't assume that you can do this with any adjective in any situation and have it seem natural. In other situations adjectives turned into nouns can seem archaic, or offensive, or ungrammatical.

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Definitely not a typo.

To me, a hypothesis is only used in academia, or if the speaker is predicting a result or the cause of a result. A hypothetical is less formal, and means a theoretical scenario.

For example: “What would you do if you had a million dollars?” is a question about a hypothetical.

From Google Dictionary (2a):

a hypothetical proposition or statement.

Source: Native AE speaker

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