Most often this is the one scene that sets the story off in the mind of the author. The scene will haunt him/her until he writes it down. Once the scene is in place, the writer can think about the various backstories that might have led the character to this particular scene.
Auxiliary verbs form the mood or tense of the other verbs in the sentence. There are three primary auxiliary verbs in the English language and these are: be, have and do.