"One thing that can make it difficult to find your ancestor is that he may have been using a different surname from the one that you expect. You will need to make yourself aware of any "dit" names that might be associated with the surname you're tracing, and if you can't find someone under the name of his child, you may find him under the dit name.
"Dit" in French means "say" and in this context, it means "called." In other words, a person might be Pierre Bourbeau dit Lacourse, which means that he had an ancestor named Bourbeau, but he chooses to use the name Lacourse instead. So he is Pierre Bourbeau called Lacourse.
People might take a dit name to distinguish their family from another family of the same name living nearby. Often it was a sort of nickname, often picked up during service as a soldier. Or it might refer to the place in France where the family originated. Sometimes it was the mother's surname, and sometimes the father's first name was used, either instead of the surname (for example, Hebert dit Emmanuel) or in addition to it (Jeanbard, Castonquay). In any case, very often the dit name was passed down to later generations, either in place of the original surname, or in addition to it.
Some of his children might then keep the original surname (e.g. Barbeau), and some might use the dit name (e.g. Lacourse). After a few generations, it's not uncommon to completely lose the memory of the original name, or to forget which was the original and which was the dit name...."