One of the things everyone remembers about Shakespeare, whether they spent a few weeks on one play in high school or an entire semester on several plays in college, is that he wrote in iambic pentameter. Some may also have vague recollections about their teacher explaining that iambic pentameter isn't difficult to understand, because English "naturally" falls into its rhythms. Dismissing iambic pentameter as merely the answer to a trivia question does a disservice to the metrical devices of both Shakespeare and colloquial English. Metrical matters inform not just poetry but the English stress rules that we use every day without being aware of them.

The "pentameter" in "iambic pentameter" refers to the number of units, or poetic feet, in a line of poetry, in this case five feet per line. The "iambic" talks about the structure of those individual units. An iamb is a two-syllable foot, an unstressed syllable followed by a stressed syllable. Iambs are based on syllables, not words, so they can cross word boundaries. Thus, iambic [entameter is five unstressed/stressed units in a row, which make up one line. This is the pattern that Shakespeare used most often in his poetry, but it should be noted that there are several other metrical patterns mixed in. A contrast between the opening lines of two plays shows this well:

If music be the food of love, play on.Twelfth Night

Now is the winter of our discontent made glorious summer by this son of York.  —Richard III

Twelfth Night begins with Count Orsino just talking (he talks a lot). He's lounging around, and there is nothing particularly urgent or exciting about his speech. This "normalcy" is rendered in iambic pentameter, with the first iamb consisting of an unstressed "if" and a stressed "mu" of "music." This is one way iambic pentameter can be called imitative of normal speech. It is not something from a poet's bag of tricks that puts the stress on the first syllable of music. That's just how the word is pronounced.

On the other hand, we have The Tragedie of Richard the Third, which begins quite differently, shoving you right into the main character's miserable brain and sharp wit with a stressed "Now."  A two-syllable poetic foot with a stressed syllable followed by an unstressed one is called a "trochee."  The first foot in Richard III is trochaic.   

People often struggle to hear the stress patterns in the context of a long, unfamiliar, Shakespearean line. There are all sorts of distractions — like plot, and character and maybe some vocabulary — that one doesn't understand. Luckily, modern English provides contrasts limited to single words which can help you hear the difference. English has a bunch of words whose part of speech (and meaning) change based only on which syllable is stressed — in other words, whether they are iambs or trochees. Here are a few:

  • permit: The verb, "to allow," is an iamb. The trochee is the piece of paper that grants a right, like a "learner's permit."
     
  • object:  The iamb is a verb meaning "to protest or express disagreement." The noun, meaning "a thing, an entity," is  trochaic.
     
  • record:  To record means "to preserve something, like a voice or music on some medium" and it's an iamb. The trochee is the noun result of that recording, like all the notes from your doctor visits being preserved in a "medical record."

In addition to these noun/verb contrasts, you also have other contrasts best exemplified by "green house" and "black bird." A "green house," written as two words, is an iamb, and can only refer to a house painted green. A "greenhouse," the trochaic, one-word version, is a glass structure where plants and flowers grow. Similarly, an iambic "black bird" can be any bird that happens to be black, but a "blackbird" is a specific type of bird and is a trochee.

With these clear differences in mind, perhaps the poetic line will be easier to handle. It's important to remember that poetic devices don't spring up fully formed to frustrate unwilling English students. Meter and rhythm and rhyme are present in our language already. Shakespeare invented a lot, but this part of the poet's art consists of taking these raw materials they have on hand and making something wonderful out of them, like a sculptor who sees the final creation in an untouched slab of marble. The building blocks of poetry are all around, present in the words we use every day.