Can a couple start their pre-Cana classes before an annulment is granted? Even if it is a defect of form? (The first marriage was performed by a justice of the peace.)
Without more information, I am unsure of the question you are asking. I assume that what you want to know is if a man and woman who plan to get married, once one or the other of them has an annulment granted for a previous marriage, can start pre-Cana instruction even though they do not yet have the declaration of nullity. (If this is not your question, feel free to re-submit it with clarification.)
The Church presumes that anything that looks like a marriage occurred is valid and/or sacramental until proven otherwise by a marriage tribunal. Such proof of nullity cannot be said to exist until the tribunal has granted the annulment. Even in the case of an obvious defect from form, the Church will want to look at the case and declare it invalid in order to ensure the freedom to marry of the individuals involved. In other words, until an annulment is granted, the individuals petitioning for the annulment must presume that they are still married to each other in the eyes of the Church.
Frankly, what this means is that someone who does not yet have an annulment for a previous marriage is not free to contemplate marriage to someone else. He or she should not even be engaged, much less preparing for marriage through pre-Cana instruction. The new couple must await the annulment decree before they can be assured that they are free to marry each other and may therefore plan their marriage.