To Martin:
In my ESL class, people have difficulties with all sorts of things. "F" becomes "p", "i" becomes "a", etc. "Not" may easily become "nat" for people, especially if they are used to their own regional dialects, not English. My students listen to me, see my mouth, and still are unable to prounounce things sometimes. It takes time and practice to get it right.
The point is that we're all here to learn together.
Practice, practice, practice. :)
Maureen.