A verb form that functions as a noun is called a gerund, such as 'eating' in the sentence 'Eating an apple a day can keep the doctor away.' The form of a word is static, but its function, such as noun, adjective, verb, or adverb, may change in context.
A verb form that functions as a noun is called a gerund. An example of this can be seen in the sentence, "Eating an apple a day can keep the doctor away, or so they say." Here, 'eating' is the verb in its -ing form, but it operates as a noun because it serves as the subject of the sentence.
Another example is found in the question, "Is this the picture of Jose's mother?" In this case, 'Jose' is a proper noun and by adding the possessive 's', it functions as an adjective, or more technically, it is functioning adjectivally to describe the mother in the photograph.
Understanding how the form and function of words can differ is essential in grammar. The form of a word, such as a noun, adjective, verb, or adverb, is static, but its function can change depending on how it is used within a sentence. Gerunds are a perfect illustration of this principle, blending the form of a verb with the function of a noun.