In Mishpatim 23:4-5, why is there a makaf in כי-תראה but not in כי תפגע?
The words are very similar. As far as I can tell the only difference is that the patach at the end of תפגע ends in an ע, while the segol in תראה is open.
My thoughts are along the lines that normally, a תנועה קטנה that has the trop on it is open. Since in תפגע the patach is closed, it becomes stronger, the same way that a תנועה גדולה that has the trop is usually closed. This pulls some of the stress away from כי, and so the makaf "snaps" (I imagine a rubber band, I don't know the technical language) and כי becomes its own word.