OK. But you have to wean yourself off solving problems by getting examples from somewhere. This is not always going to be possible.
On second thoughts I'll try just once more. If you don't get it after this I'll deliver you the solution. I do understand your frustrations, I've been there; I remember debugging for two solid weeks once and finding the problem was a missing semicolon or something daft.
The outer while loop reads strings from the user and breaks them up by spaces.
The inner loop loops over the resulting string, which is <"go> on the first time through, <4> on the second and <expert"> on the 3rd.
The string "4" contains one character (a '4') at  and a terminating null at . pos loops over this string and is the index of the character you're currently interested in. Why exactly do you think this needs to be 4? t.size() is only 1, so if pos=4 then the expression pos<t.size() is going to evaluate FALSE immediately. To go into the loop pos MUST be less than t.size(). It must also be non-negative, of course. So what is the single value that satisfies that? What number is >=0 and <1?
Since pos contains the wrong value immediately before the inner while and is fine before that, I'd say the solution probably has to go...
// <-- ...HERE