I woud also strongly recommend, dear anirbanmahato, not to count just on onine links and resources, but also to please find and read good books on English literature and its language/linguistic development. There are many fine books and am sure your teachers can recommend dozens, but id also suggest that for further details, youd do well to find and read Boris Ford's (Ed) ''History of English Literature'' Vols 1 and 2. This would give you good details yet in a simple, direct and well laid out manner.
Am giving the new revised version of this series (pub Pelican) link below, good luck!
Circa 1066 onwards, when William the Conqueror (Duke of Normandy) conquered the area that is now termed 'England', a number of peoples or tribes lived there, most prominently the Angles and Saxons, who used the Angle and Saxon languages (Germanic origins); whereas the Norman/French conquerors used Norman-French , which became for quite some time thereafter the 'official' and dominant language of the land.
However, in due course of time, Both Saxon and Norman-French combined and fused to create a new language, 'Englishe' or 'English'-- thus, its evolutionary stages coveerd Old English, Middle English and Modern English, basically. prior to this, the various people inhabiting this area never really thought of themselves as one nation but as many people/s tribes etc, each with their own culture, language, ways and so on. However, it would be right to say that as 'English' gardually evolved as a language and it eventually became the language of common and daily use, by the 12th and 13th centuries, a sense of nationhood, of national identity linked to one land and one language began to emerge. Thus, in a sense, 'England' and 'Englishmen' were born, or came into being. This process took a long time indeed, and we can note that even as late as Tudor times, the process was going on, and was probably hastened by England's wars against Spain at that time.
This last seems a very long confusing answer