1) I want to visit Fred.We can see the same phenomenon in the sentences in (3) and (4):2) I wanna visit Fred.
3) Who do you want to visit?However, no matter how casual your speech becomes, there are some cases where you can't substitute wanna for want to. In a sentence like (5), for example, want to cannot be contracted as in (6). (* is the symbol used by linguists to mark ungrammatical sentences.)4) Who do you wanna visit ?
5) I want Jim to visit Fred.Here's a pretty obvious characterization of the ungrammaticality of (6):6) * I wanna Jim visit Fred.
7) Want and to can only contract to wanna if no words are between them.Unfortunately, it's very easy to find a counter-example to (7). (9) is one, but start by looking at (8), the careful-speech version.
8) Who do you want to visit Fred?In (8), you are being asked who should be Fred's visitor. To put it another way, in (5) who questions the object of visit, while in (8) who questions the subject of visit. Here's a more graphic representation of the same point:9) * Who do you wanna visit Fred?
10) Who do you want to visit ___? (can have wanna)In each case, the gap "___" shows where we interpret who. Now notice that in (11), the gap falls in between want and to, just as Jim does in (3).11) Who do you want ___ to visit Fred? (can't have wanna)
What does it all mean?
(7) is basically correct as a description of the circumstances which allow wanna-contraction, but "words" are not the only thing that block it. As far as our language faculty is concerned, the gaps which question words like who link to are just as substantial as words like Jim, even though we don't pronounce them.
Something else to think about: no one taught you that you couldn't say (9). In fact, if anyone ever said anything about wanna to you at all it was to tell you not to use it, period. Nevertheless, you have the ability to make subtle judgements concerning its distribution. How did that happen.....