I just want to know if I can use all the progressive phrases for every situation, or I have to use specific phrase for a specific situation? The apsect is a bit confusing, but I think I can learn it with more training.
Sidder, står, ligger have inherent meanings in themselves so it would be strange in some situations to use one or the other. "I am standing and sleeping", "I am lying and running"... that must be very peculiar circumstances.
Være ved at bliver means that something is becoming: "It is getting dark". So you wouldn't use it when you are just in the middle of something like running.