tie down vs restrain

tie down and restrain both are verbs.

Word	Noun	Adjective	Verb	Adverb
tie down	No	No	Yes	No
restrain	No	No	Yes	No

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

tie down: secure with or as if with ropes
restrain: to close within bounds, or otherwise limit or deprive of free movement

Other hypernyms of tie down include confine, constrain, hold.

tie down (verb)	restrain (verb)
restrain from independence by an obligation	prevent the action or expression of
secure with or as if with ropes	to close within bounds, or otherwise limit or deprive of free movement
	restrict (someone or something) so as to make free movement difficult
	prevent (someone or something) from doing something