ruin vs burn down

ruin is a noun but burn down is not a noun.

Word	Noun	Adjective	Verb	Adverb
ruin	Yes	No	Yes	No
burn down	No	No	Yes	No

As verbs, burn down is a hyponym of ruin; that is, burn down is a word with a more specific, narrower meaning than ruin:

ruin (noun)	burn down (noun)
destruction achieved by causing something to be wrecked or ruined
a ruined building
an event that results in destruction
failure that results in a loss of position or reputation
the process of becoming dilapidated
an irrecoverable state of devastation and destruction

ruin (verb)	burn down (verb)
fall into ruin	burn completely; be consumed or destroyed by fire
destroy completely; damage irreparably	destroy by fire
reduce to ruins
deprive of virginity
reduce to bankruptcy
destroy or cause to fail