English derives from the Germanic language family: several tribes from the Continent the Angles, the Saxons, and the Jutes -- arrived in England in the fifth century C.E. and took their language with them. Soon their language was distinct enough to warrant a new name; it's now known as either Anglo-Saxon (from two of the tribes) or Old English (abbreviated OE). Although students of Anglo-Saxon can see the continuity with the English of today, at first sight it seems to be a completely foreign language. The most famous work in OE is the anonymous epic poem, Beowulf (around the eighth century).
OE was spoken in the early part of the Middle Ages, but with the Norman Invasion in 1066 came a strong French influence on the English language. Over time, the language reached the state now known as Middle English (or ME). ME, to the untrained eye, looks a bit more like English, but beginners would do well to start with a glossary.
By around the year 1500, the language had reached the stage known as Modern English (ModE). Even the first few centuries' worth of Modern English can be difficult for beginners, as anyone who's read Shakespeare can attest. But the structure of the language has remained the same for the last half millennium, with only some surface changes. Words like "dost" and "hath" lasted a few more centuries, but by 1750 were pretty much archaic, except perhaps in religious language.
(We probably associate the language of the Bible with this early stage of ModE "thou shalt not," "who art in heaven" -- because the most important English translation of the Bible, the King James Version or Authorized Version, appeared in 1611, toward the end of Shakespeare's career.)
English spelling was fluid for a long time: it settled down only in the middle of the eighteenth century, around the time when Samuel Johnson published the first major Dictionary of the English Language (1755).
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.