You are welcome to answer questions posted in the Ask a Teacher forum as long as your suggestions, help, and advice reflect a good understanding of the English language. If you are not a teacher, you will need to state that clearly at the top of your post. Please note, all posts are moderated by our in-house language experts, so make sure your suggestions, help, and advice house the kind of information an international language teacher would offer. If not, and your posts do not contribute to the topic in a positive way, they will be subject to deletion.
I sat on my suitcase as the Swiss train carried me to a meeting I'd dreamed of for two decades. At the end of the journey waited Anne Frank's father, Otto, with whom I had corresponded since I was fourteen.
So I guess, I had corresponded since I was fourteen = I used to write letter to Otto and in return they write letter. But this process occurred only after when I reached fourteen.
I had corresponded since I was fourteen = I used to write letter to Otto and in return they write letter. But this process occurred only after when I reached fourteen.
meaning 
Linear Mode
