What part of speech is “one day”?

The phrase “one day” functions as an adverbial phrase in a sentence, specifically modifying a verb, adjective, or another adverb to indicate when something happened, will happen, or could happen. Here’s a breakdown of its parts:

• “One” is a numeral or pronoun, referring to a specific, singular day.
• “Day” is a noun, referring to a 24-hour period of time.

Together, the phrase “one day” is commonly used to describe a future or indefinite time, often conveying the sense of an event happening at some point in the future or in the past.

Examples:

1. As an adverbial phrase:
   • “I hope one day I will travel the world.”
     (“One day” tells us when the action of traveling might happen—at some unspecified time in the future.)
2. In storytelling:
   • “One day, the hero found the treasure.”
     (“One day” sets the time frame for when the action occurred—suggesting a vague or distant point in the past.)

In these examples, “one day” modifies the verb (“hope,” “found”), answering the question “when?” or “at what point in time?”