* Adjective: If "first" describes a noun, it's an adjective.
* Example: "The first runner crossed the finish line." ("First" describes the noun "runner")
* Adverb: If "first" modifies a verb, adjective, or another adverb, it's an adverb.
* Example: "They arrived first." ("First" modifies the verb "arrived")
* Other: "First" can also be part of a compound adjective, like "first-class" or "first-rate".
To determine the part of speech, look at the word's function in the sentence.