grammatical differance using signifacant and significative

I had a student ask me what the grammatical rule was for using either significant and significative in a sentence. I was unable to offer a direct answer since the have similar meaning. I hope someone can offer an answer. Thanks

I have practically never heard anyone use "significative" outside the most formal of writing, and even there I have not heard of any case where you could not just use "significant" instead. Hence I would say "significant" not only when in doubt, but whenever you somehow have a very good reason to specifically use "significative" (that is, practically never).

To be honest, I've never heard of "significative", even after having taken two years of college courses.

significant:important, noteworthy; having a particular meaning

significative: being a symbol or sign of something; having a meaning

They are slightly different. And significative is really rare. I hope the definitions above would help you. If more detail is needed, let me know.