Why did they refer to worshipping Hashem as 'false matters'?
There is no reason as to why hard work is more correct and true than worshipping Hashem. But the only way to justify their demands that Yisrael relinquish the idea of worshipping Hashem was by degrading it and calling it false.
What is the meaning of "Al Yish'u b'Divrei Shaker"?
Rashi: It means 'Do not think and speak about false matters.' 1
Rashi: It does not mean, 'Do not turn to false matters' (like we find in Bereishis 4:4, in connection with Kayin), because then it would have said 'v'Al Yish'u El Divrei Shaker.'
Why did Pharaoh command to make the work harder?
Mesilas Yesharim (Perek 2): He intended that they have unceasing work, lest they have time to take counsel against him. 1
Mesilas Yesharim: So the Yetzer ha'Ra is a great crafty warrior. It is impossible to be saved from it, except through great Chochmah and supervision.
QUESTIONS ON RASHI
Rashi writes that the meaning of "Al Yish'u" is unlike the word' Sha'ah' regarding Kayin. But this contradicts Rashi to Bereishis 4:4, where he writes that it means 'to turn,' just like "Al Yish'u"!
Da'as Zekenim, Hadar Zekenim: Rashi there does not mean that it is like "Al Yish'u" here, but rather, like the verse "Al Yish'u El ha'Mizbechos" in Navi, 1 where it indeed means 'to turn.'
Gur Aryeh: Even if the term "Yish'u" does mean 'to speak' in this verse, why did Pharaoh use this unusual word, rather than 'Yedaberu'? The word choice indicates an additional level of meaning - 'let them not turn.'
See Yeshayah 17:8.
