I find the problem is actually this:  students will
have picked up the "wanna" and "gonna" verbalizations
(for "want to" and "going to") either through their
own perception or from being taught.  But whereas
native speakers use these when speaking in a rapid,
more casual style, the ESL speaker uses them all the
time (probably the ESL speaker only has one style of
English, anyhow).  So this person will be speaking
clear, careful English, usually with some formal
language, and then throw in "gonna" and it sounds very
odd.  I am not talking about people in my classes so
much as more advanced speakers I have heard.  When I
taught ESL/EFL I too taught these as  forms to
recognize, not necessarily to emulate, for the same
reasons mentioned: that you don't want to make it any
harder for the native speakers to understand you.
 


 
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