I still have some confusion as to when the Torah uses et instead of im and im instead of et. As in modern hebrew there seems to be no difference in usage when one tries to describe something is ’with’ something else; but within Torah, at least to the Vilna Gaon and Malbim commentaries, there seems to be a small nuance between the two.
Shalom! I must add, to complete the picture, that, as opposed to the Vilna Gaon, the Malbim and kabbalists, who claim that there is no such thing as two synonymous words in Hebrew, the Radak and many of the other classic commentaries have no problem saying there are synonyms. Accordingly, they feel that "im" and "et" can and are used interchangeably, and there is no significant difference between them.