It will come as no surprise that the meanings of substantive and substantial are very close.
Etymologically they are very closely related:
late 15c., "standing by itself," from O.Fr. substantif, from L.L. substantivum, neut. of L. substantivus "of substance or being," from substantia (see substance). The grammatical term (late 14c.) was introduced by the French to denote the noun in contradistinction to the adjective, from L. nomen substantivum "name or word of substance."
mid-14c., "ample, sizeable," from O.Fr. substantiel (13c.), from L. substantialis "having substance or reality, material," from substantia (see substance). Meaning "existing, having real existence" is from late 14c.
Both words derive from substance:
c.1300, "essential nature," from O.Fr. substance (12c.), from L. substantia "being, essence, material," from substans, prp. of substare "stand firm, be under or present," from sub "up to, under" + stare "to stand," from PIE root *sta- "to stand" (see stet). A loan-translation of Gk. hypostasis. Meaning "any kind of corporeal matter" is first attested mid-14c. Sense of "the matter of a study, discourse, etc." first recorded late 14c.
So we can differentiate them only through their suffixes, as follows:
sufix forming adjectives from verbs, meaning "pertaining to, tending to," in some cases from O.Fr. -if, but usually directly from L. -ivus. In some words borrowed from French at an early date it has been reduced to -y (e.g. hasty, tardy).
suffix forming adjectives from nouns or other adjectives, "of, like, related to," Middle English -al, -el, from French or directly from L. -alis (see -al (2)).
The two suffixes are pretty closely related, but in common usage, -al generally suggests a stable state, whereas -ive suggests motion towards or metamorphosis.
As far as I can tell, this analysis helps very little in deciding which of these words to chuse in a given situation. Indeed, most dictionaries list them as virtually interchangeable.
Urban dictionary doesn't list either word.
Webster and Wiktionary list substantial under substantive, either as a synonym or as a meaning, but do not list substantive under substantial.
I suspect that this might corroborate my feeling that substantive is quite often used when substantial ought to be, that is as a vague expression of quantity.
As close as I am able to get to defining a difference between these two words, it is as follows:
Substantial is used when referring to a vague quantity that the speaker things is significant in some way:
"The shop made a substantial profit this year."
Substantive is used when referring to a quality of possessing substance - either concreteness or reality:
"The change of government policy will result in substantive changes in spending." In this example, nothing is being said about the size of the changes in spending; it is stating that there will be real changes.
However, as I have already noted, it is increasingly common to find substantive used in place of substantial. I suspect this may be out of a desire to sound less vague or more educated, or both. In the former aim, it succeeds, simply because less people know the meaning of substantive, and consequently assume it has a more precise meaning than substantial.
Generally, I would advise avoiding both words. Substantial because it is too vague, and substantive because it is both too obscure and too often abused.
"The shop made a satisfactory profit this year." is much more communicative.
"The change of government policy will result in changes in spending." is much more accessible.