crouch vs sit down

crouch is a noun but sit down is not a noun.

Word	Noun	Adjective	Verb	Adverb
crouch	Yes	No	Yes	No
sit down	No	No	Yes	No

As verbs, sit down is a hypernym of crouch; that is, sit down is a word with a broader meaning than crouch:

crouch: sit on one's heels
sit down: be in a position in which one's upper body is largely upright and supported by one's backside

Other hypernyms of crouch include sit.

crouch (noun)	sit down (noun)
the act of bending low with the limbs close to the body

crouch (verb)	sit down (verb)
sit on one's heels	show to a seat; assign a seat for
bend one's back forward from the waist on down	be in a position in which one's upper body is largely upright and supported by one's backside
	take a seat