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:
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.