The spelling stayed. The sound moved on.
English has silent letters because pronunciation changes, words arrive from other languages, and some spellings were altered to show a word’s roots. There is no single explanation for every quiet letter.
A silent letter is written without a sound of its own in that word. Think of it as a guest who is on the list but never joins the conversation.
Knight brought a k it no longer uses
Knight sounds like night: its opening k is silent. You will find the same quiet beginning in familiar words such as knee and knot.
Merriam-Webster explains that these opening sounds were once pronounced. Speech changed; the written letters stayed. The dictionary’s pronunciation of knight gives no separate k sound.
Debt has a different excuse
The b in debt is silent too, but it is not simply an old English sound that faded away.
Merriam-Webster traces it to a spelling change. The word came through forms such as dette, without a b. Scholars added the letter to reflect the Latin debitum. English kept the new spelling without adding a b sound.
Sometimes a relative speaks up
Oxford University Press suggests looking at related words. The g is silent in sign, but you hear it in signature. That connection can make the spelling easier to remember.
Sounds and letters play different games. If knight and night amused you, meet some more words that sound the same but mean different things.