I’m not saying you can’t use more than one tense; the whole purpose of tenses is to enable a writer to deal with time in its various gradations, from the past to the hypothetical future (“When I telephoned my mother from the Boston station, I realized that if I had written to tell her I would be coming she would have waited for me”). But you must choose the tense in which you are principally going to address the reader, no matter how many glances you may take backward or forward along the way.
