Live language change

In college linguistics I learned that "gonna" is a word, distinct from ''going to.''   How do you prove that?  It has rules that are different from ''going to.'' For example, you can say "I'm gonna swim" but you can't say "I'm gonna the pool."  The upshot is that "going to" can be used to describe motion or the future tense, but "gonna" is specific to the future tense.  

I just found another one: ''have.''  Take this question: ''How many did you have to eat?''  It has to possible interpretations: (a) ''How many did you eat?'' and (a) ''How many were you forced to eat?''  But a speaker of vernacular English will pronounce ''have'' differently depending on which meaning they intend.  For the first a speaker will say ''have'' with the ''v'' like a ''v.''  For the second they will say ''have'' like ''half.''  Our language is changing before our very tongues.