First, I suggest you say "a phrase that I had written down." To pen is generally used, if at all, to refer to creative writing rather than to the act of writing per se.
Note in your third sentence that you meant to type prounoun, not pronounce.
You can write either you are going to do it or you will do it in your sentence. Will is, I think, a little more imperative than are going to.
Student or Learner