The Vikings after almost conquering France were given Normandy to live in then invaded and conquered England. The lords were Normans who spoke French and scandinavian (Viking). An example is the common name for cattle was cattle but when you went to sell them to the French lord they became beouf or beef.

When I was in my early twenties I lived in France for three years. I went to school to learn French with no great success. I had one private teacher who became a little frustrated with my limited success (I really was trying). He then said something that truly put it all in perspective that I as an English speaker could not understand initially. He said "when you speak French speak it beautifully, if you are not sure of how to pronounce a word say it slowly out loud as many ways as you can, the one that sounds the most beautiful is probably correct".

Yes. There are reasons.

Perhaps some of this has to do with the Great Vowel Shift...

"The Great Vowel Shift was a major series of changes in the pronunciation of the English language that took place primarily between 1350 and the 1600s and 1700s, beginning in southern England,[1][2] and today having influenced effectively all dialects of English. Through this vowel shift, all Middle English long vowels changed their pronunciation. In addition, some consonant sounds changed as well, particularly those that became silent; the term Great Vowel Shift is sometimes used to include these consonant changes as well."
[en.wikipedia.org]

Spelling was Very fluid up until the invention of "moveable type" (Gutenburg Bible, for example) and continued to be quite fluid up until the 1800's. Remember seeing "s" written as "f"?

In the case of truly English words and placenames (those that do not come from another modern language, such as cabaret and debut), many letters have been dropped simply because it's easier to say the word without them. In common with many old place names, Lincoln's name changed several times over the centuries as new languages appeared in Britain and were gradually absorbed into the mish-mash we now call English, then changed even more as English evolved: the city started out shorter as Lindon and was then extended when it fell under Roman rule, becoming latinised and lengthened to Lindum Colonia, then in Old English it became Lindocolina (and IMO, it's a shame it didn't keep that name) and later still Lincylene. At some point, the last e was dropped and the name developed into something close to the modern spelling, but was probably pronounced "link culn". There are numerous places in England named Colne, in which the the l is still pronounced, but the added syllable in Lincoln makes it harder to pronounce that way - say "link culn" over and over again and, before too long, it starts coming out as "lin cun". You can see a similar develop with the words sawyer and lawyer - pronounce them "saw yer" and "law yer"and repeat over and over and you soon start saying "soyer" and "loyer".

Psychology, of course, comes from the Greek word psyche, the whole formed by the subconscious and conscious mind, of which psychology is the study.

most such words come from old german, and the silent letters were at one time actually pronounced. notable exceptions among your examples are cabaret and debut, which comes from french, and the rules of pronunciation are different again in that beautiful language. those are not even considered silent letters; they are considered in combination to create a new sound.

g

I think it mostly has to do with the origin of the word. 'Cabaret' and 'debut' are French words for instance. 'knee' has Germanic roots and you can see in the German word 'Knie', which has the same meaning, the 'k' is not silent.