sooner vs earlier

sooner is not an adjective while earlier is an adjective.

Word	Noun	Adjective	Verb	Adverb
sooner	No	No	No	Yes
earlier	No	Yes	No	Yes

As adverbs, sooner and earlier are synonyms defined as:

sooner (adjective)	earlier (adjective)
	(comparative and superlative of `early') more early than; most early

sooner (adverb)	earlier (adverb)
comparatives of `soon' or `early'	earlier in time; previously
more readily or willingly	comparatives of `soon' or `early'
	before now