The auxiliary verb "will" functions primarily to form the future tense in English. It is a modal auxiliary verb used before the base form of the main verb to indicate actions or events that are expected to happen in the future. The basic formula for the simple future tense is: will + [root form of the main verb] For example:

• "I will learn a new language."
• "She will read that book."
• "They will sleep until noon."

Beyond simply marking the future, "will" can also express:

• Predictions or beliefs about the future ("I think it will rain tomorrow.")
• Offers, promises, or willingness ("I will help you.")
• Spontaneous decisions made at the moment of speaking ("I'll call a taxi.")
• Habitual behavior or typical tendencies ("He will give up if he starts losing.")

In negative sentences, "will" combines with "not" (won't) to indicate something that will not happen. In questions, "will" is inverted with the subject ("Will you come?")

As a modal auxiliary verb, "will" does not change form according to the subject and always requires a main verb in its base form without "to."

In summary, the function of "will" in forming the future tense is to signal that the action or event is going to take place after the present moment, often conveying certainty, intention, or prediction about the future.