Why is it seems that when you see some examples of an active to passive transformation case, some parts seem to be left out, giving an impression that it is optional? I think those in parentheses are either allowed to left out or left to give an impression that it is optional.
I didn't use any parentheses in my discussion of how to form the passive because I wanted to show how to do it using all the elements of the active structure in the passive structure. Nevertheless, a sentence does not cease to be passive just because the by phrase is left out. It's still a passive sentence. So sometimes passive sentences are illustrated with parentheses around