Changes in spelling from Old English to modern English.

One interesting thing to add, of course, is that a number of the silent letters in Present Day English -- such as the "k" in "knife" or "knight" -- are fossils. Those letters were once pronounced, and many words with silent letters can be traced back to Old English. (One exception is the final silent "e" at the end of words such "knife"; in most cases, this final "e" never was pronounced)

According to the Online Etymology Dictionary (see the link below), the word "knife" in late Old English was spelled cnif ("c" and "k" often represent the same sound). Similarly, in Old English the word "knight" was spelled cniht. (The word meant "boy, youth, servant" and hadn't yet taken on the meaning of the cultured, armored man on a horse). The "h" in cniht was probably a gutteral "ch" sound similar to the sound in the German word "ach"; that "h" is latter spelled with "gh" (probably with little to no change in pronunciation), and we still use "gh" to spell the word, but those letters are just as silent today as the "k" at the beginning of the word.

Your question, of course, seems to ask for a survey of all of the spelling changes from Old English through Middle English and Early Modern English into Present Day English. As the previous poster said, there are a lot of changes. I would recommend consulting a textbook on the history of the English language (such as C.M. Millward); in any good textbook you'll find more than you want to know about these sorts of changes.