I sometimes describe parts of English spelling as “a museum of earlier pronunciations.”
This doesn’t explain all the idiosyncrasies of English spelling, of which there are many, but it does explain some of the silent consonants we still see today.
English used to have some consonant clusters at the beginnings of words that we no longer say as clusters because one sound has been lost (lost in the spelling and pronunciation or lost just in the pronunciation). For example, Old English (spoken before the year 1100) featured the initial consonant clusters “hl,” “hr,” “hn,” “gn,” and “kn.”
To give you some examples, the word loafin Old English was hlaf.The Modern English word ringwas hringe.And the word neckwas hnecca.The “h” sound at the beginning of these words was dropped fairly early in the history of English; and importantly, it stopped being pronounced before spelling was standardized. As a result, in the spelling of these words, we no longer see the “h.”
In contrast, let’s take the words gnatand knife.The consonant clusters at the beginning of both words used to be pronounced. The initial “g” and the initial “k” were lost during the Renaissance right as spelling was getting standardized. Thus, the spelling preserves the sound even though we’ve lost it in the pronunciation.
This fact also explains a word like knight,in which both the initial “k” and the medial “gh” were pronounced. (The “gh” represented a sound linguists call a palatal fricative: an “h”-like sound pronounced with your tongue moved up to or toward the hard palate at the top of your mouth.)
Now, earlier pronunciations don’t explain all the silent consonants in modern spelling, and I want to give you one example, which is the “s” in island.
The “s” in island is a mistake.
The word islandgoes back to Old English, to a compound word that meant “water land” or “river land.” In Old English it was often spelled iglandor iland,and the word was still spelled ilandin the 15th and 16th centuries. But in the Renaissance, the word came to be associated with the French-borrowing isle,which goes back to the Latin insula.As a result, scholars thought the “s” had gotten left out somewhere in the history of English, and in the Renaissance they put the “s” back in.
As these examples demonstrate, sometimes silent consonants capture an earlier pronunciation in the history of English and sometimes they capture an earlier misunderstanding.