burden vs bear down

burden is a noun but bear down is not a noun.

Word	Noun	Adjective	Verb	Adverb
burden	Yes	No	Yes	No
bear down	No	No	Yes	No

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

Other hyponyms of burden include overburden, deluge, flood out, overwhelm, adjure.

burden (noun)	bear down (noun)
weight to be borne or conveyed
the central idea that is expanded in a document or discourse
an onerous or difficult concern
the central meaning or theme of a speech or literary work

burden (verb)	bear down (verb)
impose a task upon, assign a responsibility to	exert a force or cause a strain upon
weight down with a load	pay special attention to
	exert full strength
	to make a rush at or sudden attack upon, as in battle
	exert a force with a heavy weight
	contract the abdominal muscles during childbirth to ease delivery