By enabling the Subdivision modifier, the newly generated vertices are twisted in a weird way as shown on the screenshot. Why?
Reason: Although points D and C are connected by an edge, the points ABCD do not form a face yet, and the points CDEF do not form a face either. Instead, the points ABEF do form a larger face.
Question: Is there any quick way to form faces for the points ABCD and the points CDEF, and delete the face for the points ABEF please? (The less number of mouse clicks are required to achieve this goal, the quicker and the better method it must be.)
BTW, on the screenshot 2, only 2 vertices are selected. After enabling the Subdivision modifier, 14 vertices are highlighted automatically on the screenshot 1. This is because the addon called "Draw Xray" by bartoszstyperek was turned on. The addon does not affect the end result, and is supposed to only provide a better visual experience in the viewport.