Why does English have so many words that start with “kn”, but the K is silent and it’s pronounced “n”. My almost-1st-grader would like to know.
Don’t know (ha!), but it’s so ingrained in our language that it took me months to get my wife to stop mispronouncing Knik, wherein both 'k’s are pronounced. Words like that are common to the Native language in Alaska.
The k was once pronounced, as it still is in many German words, such as Knecht, knight. I think it was around the 14th century that many of these pronunciations shifted in English.
The silent K’s in “The knight knows his knives” (and the silent G in “gnome”) are relics of a time when they were sounded. IIRC that was in Middle English, around 1300, but that’s purely from memory.
Note that a lot of Latin words, including a few common names, begin with Cn- or Gn- and are sounded (in classical pronunciation) just like that. In Italianate “Church Latin” they take on a palatalized /n/ sound like a Three Stooges “nyuk, nyuk”
It goes back to Old English: cniht = young man; (ge)cnawan = to know.
In the glossary of my Bright’s Old English, we also have cneo = knee and cnittan = to bind (knit). The ‘C’ was pronounced as a hard K.
But we retain the spelling because of the pronunciation during the Middle English period when speillings were becoming standardized. Had the pronunciation changed by then, the spelling probably would’ve changed as well, dropping the silent “k”.
In his PBS series “The Story of English” Robert MacNeil points a finger at printer William Caxton.
English was going through “The Great Vowel Shift” at this point, but printers like Caxton fixed the spelling before there was a consensus.
Yeah, that Billy Caxton was always a trouble maker. 
The same is true of the “gh” at the end of words: it’s a representative of the velar fricative (like in the German “ach”) and was pronounced that way when spelling was first standardized. Thus, “knight” was prounounced with all letters having a value (the scribes uses “gh” to represent this sound; they were French, which didn’t have an equivalent, but it is a pretty good representation of what it sounded like). The velar fricative was lost, and other sounds were used in its place, leading to the different pronunciation of “-ough,” which was written exactly the way it was pronounced.