Substantial
Part of speech: adjective
Definition: Belonging to substance; actually existing; real; as, substantial life. Not seeming or imaginary; not illusive; real; solid; true; veritable. Corporeal; material; firm. Having good substance; strong; stout; solid; firm; as, substantial cloth; a substantial fence or wall. Possessed of goods or an estate; moderately wealthy; responsible; as, a substantial freeholder. Large in size, quantity, or value; as, a substantial amount of money; vast Most important; essential. Ample or full. Significantly great.
Example sentence: The factor most ignored in discussing interstellar flight is the kinetic energy that must be invested in the ship to make its tons of matter move at a substantial fraction of the speed of light.
Substantive
Part of speech: adjective
Definition: Of the essence or essential element of a thing; as, "substantive information".Having substance and prompting thought.Applying to essential legal principles and rules of right; as, "substantive law".Of a dye that does not need the use of a mordant to be made fast to that which is being dyed.
Part of speech: noun
Definition: A word that names or refers to a person, place, thing, or idea. Nouns and personal pronouns are always substantives by nature. Other pronouns, numerals and most determiners readily function as substantives in English and many other languages. When present participles function as substantives in English, they are called gerunds; many other languages prefer to use infinitives more often for this purpose. Care in translation is needed when adjectives are used as substantives, because languages differ in which ones are permitted and which ones must remain adjectives.
Example sentence: We need to work together, on a bipartisan basis, to create new jobs, increase job training, enact real and substantive middle class tax relief, and reward companies that create jobs at home.