What is meant by four-act opera

When the opera was just born, the facilities of the opera house were very simple, unlike nowadays when you can use electronic equipment to set the scene, and the curtains were drawn by hand in those days. And the composers took this into account and decided to divide the opera into acts to distinguish between different scenes.

Generally speaking, operas are divided into acts based on a wide range of changes in location and time, or the occurrence of an event that affects the development of the whole opera, for example, the first act of The Lady of the Camellias is in the city of Paris, and then three months later in the suburbs of Paris, and then in the city of Paris in the third act; the first act of La Bohème is in the workshop where the artists are living, and then the second act is a celebration of a festival outside; Aida is in the first act before going on a journey, and then the second act is a celebration of a festival outside; Aida is in the first act before going on a journey. In "Aida" the first act is in the palace before the expedition, the second after the triumphant return, and so on.

Because of the limitations of the opera itself, there are times when some of the emotions and the drama cannot be expressed as a whole, and this is where the subplots are needed to help accomplish this, and at the same time, the drama of the entire play is expressed in an orderly fashion through reasonable subplots.