English (like all other languages) isn't a pure language. It is basically a mixture, a hodgepodge of other languages which has developed and formed over time.
The basis of English was originally a West Germanic language related to Dutch and Frisian however over the years it has received considerable input from Latin, French (itself derived from Latin) and Greek as well as a great number of other languages.
Nowadays, as the de facto lingua franca of world commerce, diplomacy and suchlike, knowledge of English is incredibly important and in most countries children are taught English in state schools from an early age. Many countries also have small private schools teaching English and parents will happily pay for their children to have lessons in the hope that it will increase their chances of better employment and opportunities later in life.
The history of the language is tied up with the history of England. When Julius Caesar came to Britain about 2,000 years ago English did not exist as such. Later the forerunner of the language was brought over by Angles, Saxons, Jutes and other Germanic tribes giving rise to Anglo-Saxon or Old English in the 5th century CE, first appearing in writing in the 8th century CE. At this time Englisc was not widely spoken at all and had very little influence.
Following the Norman conquest of 1066 the language absorbed many French terms and later still with the growth of learning and education came the arrival (and absorption) of Latin and Greek vocabulary.
By the time of Shakespeare at the end of the 16th century, English was spoken by some 7 million people and had little or no influence outside the island. However this changed with the expansion of the British Empire which led to the spread English around the world and also brought into the language many new terms from foreign languages (giving rise to a number of variations of English such as British English, American English and so on).
As you can see, over time the English language has changed considerably and continues to change to this day with new words being formed (neologisms) and words imported from other languages as the need arises. Thus we cannot say that there has ever been a single time when the English language has been stable. It is not, in other words, static. In teaching English this must be remembered.
The very first example of written English comes from a set of bones inscribed in Runes and (probably) used for playing a game. They were discovered in a cave in Norfolk and date from about 400 or 500 CE. The bones are from sheep except for one which comes from a deer. This is inscribed with the rune raihan which is Old English meaning roe-deer.
Over 340 million people speak English as a mother tongue with almost 68% of those in the USA and 17% in the United Kingdom. A further 300 million speak it as a second language in more than 100 countries around the world and over a billion people speak it to a greater or lesser extent.
It is the lingua franca of business, communications, science, the internet, aviation, entertainment, radio and diplomacy to name but a few fields.
There are estimated to be well over half a million words in general English with about 25,000 being added each year. On top of this are another half a million words with specialised technical or scientific use.
See the main article, Vocabulary.