What is he history behind there being so many different pronunciations of the same letter sequences and why are there so many ways of writing the same sounding words in the English language? I speak and write three languages and have basic knowledge a three others and English stands out as the language where the written and spoken form are most at odds. For example: Though, through, threw, tough, thought, thou and there, their, dare, they're. I'd heard that it was due to the waves of different peoples moving to the British isles and the pidgin language that ensued. Is this correct (or did illiterates invent the spoken form and deaf mutes the written)?
Just a comment, hopefully permitted by the moderators - the term "deaf-mute" is considered archaic and offensive. Deaf and hard of hearing people are generally not mute at all, and have functioning vocal cords. Because modulating one's own voice is easier when you can hear it, many deaf and hard of hearing people do not speak, or do not speak clearly; this does not make them mute, as they have the physical capability to speak.
The term "deaf-mute" was used in the 18th and 19th centuries but has fallen out of favor; the term "deaf and hard of hearing" is preferred today.
Thanks for your understanding.
Part of the reason English orthography is so complex is that it reflects a fairly archaic sound system. The letter combination <gh>, for instance, reflects an old phoneme /x/ (as in German Bach) that was lost or sporadically changed to /f/. English vowel letters are pronounced differently in different contexts (e.g. mat vs mate) because spoken English has a quite different vowel system these days. A word like mat and mate originally had the same vowel. However, the vowel of mate ended up lengthening and raising. This happened throughout the English lexicon with the entire vowel system, and is called the Great Vowel Shift. It's responsible for much of the (comparatively) complex mapping of vowel letters to vowel sounds in English.
To add to what /r/rusoved said, English also has a significant amount of words of non-Anglo-Saxon origin, and when they were assimilated to the language, it seemed there was little done in the way of anglicizing or adapting the spelling to more accurately represent the English pronounciation or that the pronounciation gradually changed over time or only regionally and the spelling was not. Of course, this provided that the words were originally written in the Latin alphabet and simply borrowed mostly, 'as is' - as would be the case with French, Latin, and some Greek words via their Latin transliteration.