to-do vs incident

to-do and incident both are nouns.

Word	Noun	Adjective	Verb	Adverb
to-do	Yes	No	No	No
incident	Yes	Yes	No	No

As nouns, incident is a hyponym of to-do; that is, incident is a word with a more specific, narrower meaning than to-do:

to-do (noun)	incident (noun)
a disorderly outburst or tumult	a single distinct event
	a public disturbance

to-do (adjective)	incident (adjective)
	(sometimes followed by `to') minor or casual or subordinate in significance or nature or occurring as a chance concomitant or consequence
	falling or striking of light rays on something