English used to be a normal Germanic tongue with its four cases (plus instrumental in some pronouns), complex tenses, three numbers and three genders. At that time it was called good Old English. At the beginning of the Middle English period, which dates from the Norman Conquest of 1066, the language was still inflectional; at the end of the period the relationship between the elements of the sentence depended basically on word order. As early as 1200 the three or four grammatical case forms of nouns in the singular had been reduced to two, and to denote the plural the noun ending -es had been adopted.

The declension of the noun was simplified further by dropping the final n from five cases of the fourth, or weak, declension; by neutralizing all vowel endings to e (sounded like the a in Modern English sofa), and by extending the masculine, nominative, and accusative plural ending -as, later neutralized also to -es, to other declensions and other cases. Only one example of a weak plural ending, oxen, survives in Modern English; kine and brethren are later formations. Several representatives of the Old English modification of the root vowel in the plural, such as man, men, and foot, feet, survive also. With the leveling of inflections, the distinctions of grammatical gender in English were replaced by those of natural gender. During this period the dual number fell into disuse, and the dative and accusative of pronouns were reduced to a common form. Furthermore, the Scandinavian they, them were substituted for the original hie, hem of the third person plural, and who, which, and that acquired their present relative functions. The conjugation of verbs was simplified by the omission of endings and by the use of a common form for the singular and plural of the past tense of strong verbs.

The transition from Middle English to Modern English was marked by a major change in the pronunciation of vowels during the 15th and 16th centuries. This change, termed the Great Vowel Shift by the Danish linguist Otto Jespersen, consisted of a shift in the articulation of vowels with respect to the positions assumed by the tongue and the lips. The Great Vowel Shift changed the pronunciation of 18 of the 20 distinctive vowels and diphthongs of Middle English. Spelling, however, remained unchanged and was preserved from then on as a result of the advent of printing in England about 1475, during the shift. (In general, Middle English orthography was much more phonetic than Modern English; all consonants, for example, were pronounced, whereas now letters such as the l preserved in walking are silent).
All long vowels, with the exception of [ì] (pronounced in Middle English somewhat like ee in need) and [u] (pronounced in Middle English like oo in food), came to be pronounced with the jaw position one degree higher. Pronounced previously in the highest possible position, the [ì] became diphthongized to "ah-ee," and the [u] to "ee-oo." The Great Vowel Shift, which is still in progress, caused the pronunciation in English of the letters a, e, i, o, and u to differ from that used in most other languages of Western Europe. The approximate date when words were borrowed from other languages can be ascertained by means of these and other sound changes. Thus it is known that the old French word dame was borrowed before the shift, since its vowel shifted with the Middle English [a] from a pronunciation like that of the vowel in calm to that of the vowel in name.

In the late 17th century and during the 18th century, certain important grammatical changes occurred. The formal rules of English grammar were established during that period. The pronoun its came into use, replacing the genitive form his, which was the only form used by the translators of the King James Bible (1611). The progressive tenses developed from the use of the participle as a noun preceded by the preposition on; the preposition gradually weakened to a and finally disappeared. Thereafter only the simple ing form of the verb remained in use. After the 18th century this process of development culminated in the creation of the progressive passive form, for example, "The job is being done."

The most important development begun during this period and continued without interruption throughout the 19th and 20th centuries concerned vocabulary. As a result of colonial expansion, notably in North America but also in other areas of the world, many new words entered the English language. From the indigenous peoples of North America, the words raccoon and wigwam were borrowed; from Peru, llama and quinine; from the West Indies, barbecue and cannibal. In addition, thousands of scientific terms were developed to denote new concepts, discoveries, and inventions. Many of these terms, such as neutron, penicillin, and supersonic, were formed from Greek and Latin roots; others were borrowed from modern languages, as with blitzkrieg from German and sputnik from Russian.

In Great Britain at present the speech of educated persons is known as Received Standard English. A class dialect rather than a regional dialect, it is based on the type of speech cultivated at such schools as Eton and Harrow and at such of the older universities as Oxford and Cambridge. Many English people who speak regional dialects in their childhood acquire Received Standard English while attending school and university.

Widely differing regional and local dialects are still employed not only in the various counties of Great Britain, but also around the world in many countries which took up English. American, Irish, Canadian, Australian and Indian English are different from British English mainly in vocabulary and prononciation, though even some grammar changes exist.