What does the Pasuk mean when it says "Va'yasar Es Ofan Markevosav"?
Rashbam: The Egyptians tried to arrange the wheels to go back (some sort of gear system), but they were unable to do so.
Hadar Zekenim Hadar Zekenim #1: The sea [floor] is higher near the shore than in the middle. The Egyptians saw that the wheels are rolling them to the depth, so they removed the wheels and tried to flee from the sea. The chariots went slowly because the wheels were removed.
Hadar Zekenim #2: An angel removed one wheel from the chariot, so it went slower than if both were removed.
Bechor Shor citing R. Shmuel: "Va'Yasar" means that they wanted to veer and go back, and they were pressed against each other, and screaming 'veer from me', but they could not.
Moshav Zekenim: Hashem did so while Yisrael were still in the sea, and also made mud where the Egyptians were, to stop them from getting close to Yisrael. Only after the last Yisrael left the sea, the water covered the Egyptians. This explains the order in Emes v'Yatziv 'Yam Suf Bakata v'Zedim Tibata vi'Didim He'varta va'Ychasu Mayim Tzareihem.' The Egyptians began sinking in mud before the water covered them; R. Yehudah ha'Chasid explained differently.
What is the significance of the fact that Hashem "led them with heaviness" (bi'Cheveidus)?
This was Midah Keneged Midah, because Par'oh and his servants hardened their hearts ("Va'yachbed es Libo ... " [Rashi]). See also Ba'al ha'Turim.
If Hashem was fighting against Mitzrayim, what would they gain by fleeing?
Seforno: They thought that if they retracted and fled, Hashem would stop fighting them. 1
Why does the Torah add the word "b'Mitzrayim"?
Rashi #1: It means against the Egyptians (as if it had written 'b'Mitzriyim'). 1
Rashi #2. Rashbam: Just as the Egyptians at the Yam-Suf were being smitten, so too, were those who remained in Egypt. Hadar Zekenim - they were stricken similarly, and drowned in the Nile River.
Since the verse is quoting the Egyptians, why does it not say 'Banu'? It seems that this is so we can expound also that those who remained in Egypt were also being smitten. (PF)