Many a time I have heard people try to imitate archaic English (and by archaic I mean as it was spoken in Shakespeare's time), for the sake of a joke or something, and they obviously have no idea how to do it. They use all the wrong forms of the pronoun "thou" and don't conjugate verbs to it, and they often add verbal endings like "-eth" and "-est" to nouns. Here follows a short explanation of some of the main differences between modern and archaic English which will not only improve your Shakespeare jokes, it might help you read actual Shakespeare!

1. Second person singular pronoun and accompanying verb forms

In modern English, we have only one word for second person singular and second person plural. That is, whether you're speaking to one person or a group of people, you still call them "you". In archaic English, however, "you" (or "ye") was reserved for the plural and the singular was always "thou". Here are all the forms of that pronoun:

Nominative (subject): Thou
Objective: Thee
Possesive Adjective: Thy (sometimes "Thine" before vowels)
Possesive Pronoun: Thine
Reflexive: Thyself

When "Thou" is the subject of a verb in the present tense, you must put the verb in the second person singular by adding "-est", or just "-st" if the verb already ends in an "e". Thus "thou eatest", "thou sleepest", and "thou lovest". Some irregular verbs are "thou art", "thou canst", "thou didst", "thou dost" OR "thou doest", "thou hadst", "thou hast", "thou mayst" OR "thou mayest", "thou must", "thou shalt", "thou wast" OR "thou wert".

2. Alternate third person singular flexion

Sometimes in archaic English a verb ends in "-th" or "-eth" were we would use "-s" or "-es". Thus "time flieth", "my cup runneth over", "this sucketh". It's that simple. Some irregular forms are: "doth" OR "doeth", and "hath".

3. Simple negation

In modern English whenever we want to negate a verb we have to add an auxiliary such as "do", e.g. "I want ketchup", "I do not (or don't) want ketchup". In archaic English they added "not" right after the verb, with no auxiliary: "I want not ketchup". This is a much simpler and easier system of negation and I don't know why it went out of use.

4. Archaic English also has some cool words like "whither" and "thence"; see from whence for more

note: according to OED, "ye" was used as "subjective & (later) objective (direct & indirect): used by the speaker or writer to refer to the persons or (later, orig. as a mark of respect) single person he or she is addressing, as the subject of predication or in attributive or predicative agreement with that subject." See "Use of plural pronouns for respectful singular". Thanks PMDBoi!

another note: the "ye" you see used as an article in things like "ye olde pub" is not the same word as above, it is pronounced "the" and is only written with a "y" because of an error in typography involving the substitution of a superscript "y" for the thorn character ("Þ, þ") used to represent an unvoiced dental fricative. For more information see ye, thorn, and þ.