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.
It is hard to answer this question because the changes from Old English to Modern English have been so great that it's hard to talk about them as even being the same language
There are a couple of really major changes other than spelling changes. Old English was highly inflected, with inflections for gender, case, and number. There are also many words from Old English that no longer exist in modern English.
As for spelling, the Old English alphabet was not the same as ours. It did not have the letters v or j and did not really use q or z. In addition, it had three letters that are no longer in our alphabet: thorn, eth, and wynn.
For more on this and for some spelling rules, follow the links. The first one has some spelling and pronounciation rules, the second is more general.
Starting in the late 1300s, many English vowels began to shift to new ones. Much of our spelling reflects the stage before this shift. To understand it, we need to see how sounds fit into the human mouth. These are the basic vowels the way we learn them in, for example, Spanish:
Vowels began shifting upwards on this grid. Notice that a word such as FOOD is spelled with two o’s. It used to be pronounced "fode," but its pronunciation moved up into the "u" region and became what it is now. The spelling has stayed the same, but the language has moved on. Over on the other side of the chart, a word like FEED was originally pronounced "fade," but the sound moved upward so that now it is pronounced with the "i" sound.
While words such as FEED left their "slots," words with the ah sound of "NAH-muh" moved up and took their place. This is why the word is now pronounced "naym"—and why made is pronounced the way it is instead of the way it is spelled, MAHduh," and so on.