The examination of language in Henry IV, Part One has primarily focused on the contrast between prose and verse. Researchers have investigated how these speech forms differentiate the play's two distinct worlds. In the tavern, ruled by Falstaff, characters communicate in prose, while in the court, which also represents the historical setting, characters speak in verse. Hal, who is at ease in both settings, adapts his language form to suit his surroundings. Falstaff, who is a stark contrast to the courtly environment, speaks solely in prose. It has been noted that Hotspur delivers the play's most outstanding verse, with his speeches reflecting his character's directness and steadfastness.

The use of oaths, or promises, and the frequent manipulation of language in the play have also been examined. Oaths, often broken by various characters including Falstaff, can indicate a character’s moral standing, suggesting that those who honor their word may possess greater moral virtue. Hal learns to manipulate language, which some critics claim is an essential part of his development. He becomes adept at speaking like Hotspur when necessary, engaging with commoners, articulating like a king, and expressing his true self.