let alone vs let

let alone is not a noun while let is a noun.

Word	Noun	Adjective	Verb	Adverb
let alone	No	No	Yes	Yes
let	Yes	No	Yes	No

As verbs, let is a hyponym of let alone; that is, let is a word with a more specific, narrower meaning than let alone:

let alone (noun)	let (noun)
	a serve that strikes the net before falling into the receiver's court; the ball must be served again

let alone (verb)	let (verb)
leave unchanged or undisturbed or refrain from taking	leave unchanged
	cause to move; cause to be in a certain position or condition
	actively cause something to happen
	consent to, give permission; permit
	grant use or occupation of under a term of contract
	make it possible through a specific action or lack of action for something to happen

let alone (adverb)	let (adverb)
much less