Reading any one of Shakespeare's plays can feel like reading a really lengthy poem and that's because they're written in a combination of verse (poetry) and prose (how we talk every day). We break all of this down in the paragraphs that follow but here's what you should remember about Shakespeare's plays. The nobility tend to speak in "blank verse," which is a formal way to talk. The commoners or, "Everyday Joes" tend to speak just like we do, in regular prose. (Note: The play Richard II is the one exception to this rule – it's the only Shakespeare play written entirely in verse – even the gardeners speak poetry.)
In Henry IV noble characters typically speak in iambic pentameter. (However, Prince Hal has a tendency to speak in regular prose, especially when he hangs out with the commoners so, you'll want to keep an eye on when and where he alters his speech style.) Don't let the fancy name intimidate you, iambic pentameter is simple once you get the hang of it.
An "iamb" is an unaccented syllable followed by an accented one. "Penta" means "five," and "meter" refers to a regular rhythmic pattern. So "iambic pentameter" is a kind of rhythmic pattern that consist of five iambs per line. It's the most common rhythm in English poetry and sounds like five heartbeats:
ba-DUM, ba-DUM, ba-DUM, ba-DUM, ba-DUM.
Let's try it out on this line, where King Henry IV complains about his sleepless nights:
unEAsy LIES the HEAD that WEARS the CROWN
Every second syllable is accented (stressed) so this is classic iambic pentameter.
Over half of Henry IV Part 2 is written in prose and this has a lot to do with the fact that Shakespeare introduces so many "low" or common characters in the play. That being said, their language is far from boring. Falstaff and the rowdy Eastcheap crew tend to talk in a bawdy way with lots of punning and double entendres. If King Henry IV (who we have already established speaks mostly iambic pentameter) talks in a formal way that's befitting his position as a monarch, then the commoners speak in a way that's just as raucous and out of control as they are. Want an example? Go to "Steaminess Rating."