Here's why:
* "Foreboding" is a noun, but it's an abstract noun. This means it represents a concept or feeling rather than a concrete object.
* Simple subjects are usually concrete nouns or pronouns. For example, "the cat," "she," or "the car."
* You can use "foreboding" as part of a noun phrase. For example: "A foreboding feeling filled the air." Here, "foreboding" is part of the noun phrase "a foreboding feeling."
Example of a correct sentence using "foreboding":
* The foreboding atmosphere made everyone uneasy.
In this sentence, "atmosphere" is the simple subject. "Foreboding" is an adjective describing the atmosphere.