Shakespeare is written in Elizabethan English, highly poetic, and uses iambic pentameter.  Iambic pentameter is an unstressed syllable followed by a stressed syllable.  This can be evidenced in both his plays and notably in his sonnets.  Iambic pentameter is a form of poetic device, which lends rhythm and an almost musical quality to the language, both written and spoken. 

"Shall I compare thee to a summer's day?"  This line is taken from Sonnet 18.  I will attempt to give you an example through bold type as to the syllable stressors.  "Shall I compare thee to a summer's day?"  I am sure this is a bit confusing, and believe me when I state that learning iambic pentameter is not so easy.  Once you get the rhythm, or the feel, of the stressed syllables, this will become much easier. 

Do not make the mistake, however, of thinking that you can only read and understand Shakespeare's writing if you understand iambs.  This is just a poetic device that many authors, notable Shakespeare, have used.  Familiarize yourself with thee and thou and other such words that we do not use in our everyday speech.  Old prayers and hymns also use these words as was the style of the times in which they were written. 

Elizabethan English is considered to be a part of Early Modern English.  It was basically a transition from Middle English.  Just as all language evolves, so has the English language.  The Renaissance gave way to this type of English style. 

It is important to know that Shakespeare wrote during the reign of Queen Elizabeth I, and was highly influenced both politically and stylistically.  This is  where the term Elizabethan English stems from.  Queen Elizabeth I was rumoured to be a patroness of Shakespeare (many stories suggest other benefactors), and plays such as Richard III where politically manipulated i order to please the political climate of her rein.

Romeo and Juliet by William Shakespeare is written in English. Shakespeare's works, including Romeo and Juliet, are considered among the best and most beautiful poetry in the English language. 

The English language is normally divided into Old English, Middle English, and Modern English, according to the following criteria:

Old English or Anglo-Saxon: (ca. 450 AD to 1066 AD) was a west Germanic language spoken in England. The best known work in it is the epic Beowulf. It differs greatly from modern English in both syntax and vocabulary.

Middle English (1066 to 1480): Middle English begins with the Norman Conquest which brought many French loan words into the English language. This also marks the beginning of the Great Vowel Shift and other changes in pronunciation, simplifications of grammar, and shifts in spelling. The best known poet from the period is Chaucer.

Early Modern English (c. 1480 to 1650): Shakespeare is an example of early modern English. Although spelling was not fully standardized until the eighteenth century, Shakespeare's English is quite close to twenty-first century English in vocabulary and grammar. Although 21st century readers may occasionally need to consult a dictionary, and read carefully, most native and near-native English speakers should be able to read Shakespeare's English.

Modern English: The modern English period begins in 1650; by this date, vocabulary, spelling, and grammar begun to be regularized and the Great Vowel shift completed, making pronunciation fairly close to our current version. 

Modern English (1650 through present)