Easy. Show both the text field and the traditional UI at the same time right from the beginning and link them together so that manipulating the traditional UI updates the text field and vice-versa. Have an area where you show a few additional alternate syntaxes for the same "sentence" so that users can see that the syntax is far from being rigid, however if you input a sentence and then later manipulate the UI, the text field should retain the original style (not canonicalize to the default syntax).
Ok, this might not be so easy to implement but I'm pretty sure it's the best solution for the user as it scales from beginner to expert.