I have been trying to figure out for at least an hour a way to use the F-key but not make any diagonal faces. I just want 90 degree angles. Am I pressing the wrong button? Couldn’t find anything on the google machine…
It’s not clear what you mean by making diagonal faces at 90 degrees. Do you mean end cap made of Triangular fan?
I mean I want a 90 degree angle between the wall of the half oval and the piece that is going through the diameter. Instead of it making diagonal and sharp angles. I want it to be like a bridge between the two walls of the oval shape. I don’t know the technical terms for it 
First, it is obvious that you are using the Subsurf modifier (a.k.a. Subdivision Surface). It will always make any corner rounder if you do not add a loop cut next to the edge. If that is not enough to your liking, you can also increase the Mean Crease. (In the [N] panel or with the shortcut [SHIFT E].)
Next, because you selected more than 2 edges, you created an Ngon i.e. a face with more than 4 vertices. The Subsurf modifier doesn’t like them. It will create a bunch of tiny triangles around the edges.
Thank you! Always learning
I’m… ehrmmm… little or so lazy and we do have n-gons at hand…so, if it is what i think it is, i mean this;), then there’s no really need to stick to classics anymore. They (n-gons) are fine if isolated on flat and if there are no immediate plans to export to fancy outer world applications which do not know anything about them.
