What does the Torah imply when it notes 'the Cannannites were then in the land' (Gen. 12:6)? It seems to suggest they were in the land then (when the tale took place) but not now (when the book was written). As there were Cannannites in the land during the entire lifetime of Moses, who wrote the Pentateuch, why does the text seemingly speak in the past tense?
Rashi on the verse (Bereshit 12:6) tells us that Israel was part of the portion of Shem. At that time, the Canaanites (descendants of Cham) were conquering the land from them.
The Chumas Shai LaMorah brings a Chizkuni (to both the verses from Bereshit and Devarim cited above) that explains the southern part of Israel was given to Cham, while the nothern part was given to Shem. Apparently these northern lands were now being conquered by the Canaanites.
Two other answers are brought here:
Rav Eliyahu Mizrachi (in his commentary to Rashi in Parashat Lekh-Lekha) concludes that we have no choice but to view these two Midrashim as conflicting with one another. It should not surprise us, Rav Mizrachi adds, that Rashi adopts both views in his commentary; in any given context he adopts the explanation that he feels best explains the local text, even if it contradicts a claim made to explain the text in a different context.
Levush ha-Ora (in Bereshit). When Rashi writes in Sefer Bereishit that Canaan captured the territory from Shem, he did not mean that Canaan launched an independent campaign. Rather, his father, Cham, assigned him the task of conquering the unsettled areas of the land for him, Cham. After Canaan completed the conquest, Cham rewarded him with the land and built the city of Chevron. We can thus reconcile the two comments of Rashi. On the one hand, the land originally belonged to Shem and was seized by Canaan; however, Canaan acted not independently, but on behalf of his father, who then built for him the city of Chevron.
I'm not sure I'm understanding it correctly, but the Chizkuni (Bereshit 12:6) also brings another explanation (seemingly not according to Rashi). He says that the word "Az" is future tense, telling us that not only were there Canaanim in the land during the time of Avraham, but there will also be Canaanim in the land when it comes time for the Jews to conquer it.
יתכן שארץ כנען תפשה כנען מיד אחר ואם איננו כן יש לו סוד והמשכיל ידום.
It is possible that Canaan took the land of Canaan from someone else. And if not, then it has a secret, and the intelligent one will remain silent.
So his first answer, similar to Rashi quoted in Menachem's answer, is that the word 'then' is saying that Canaan had already, by then, conquered the land. It is not discussing the status of the land at the time the verse was written.
נראה שלא כתב משה זאת המילה בכאן רק יהושע או אחד משאר הנביאים כתבוה
It appears that Moshe did not write this word here, rather Yehoshua or one of the other prophets wrote it.