Modernism and Postmodernism are two cultural trends which developed in the twentieth century. Modernism came up first, it developed in the first three decades of the century, while Posmodernism began its development just after the Second World War, even if it became a popular trend only in the 80s. From the name of these two trends you can easily understand that they were consequential, but not only in time, in fact Postmodernism goes ideologically beyond Modernism even if it faced the same topics, that are the following: about society and social structure modernistsbelieved there was order, while postmodernists think society will develop in chaos and will lose its order; about social roles postmodernists relate on the Theory of Chaos too, and believe, despite modernists who said they were still and ordered, that each man changes his role in the society in every situation.
The most important topicof the twentieth century, anyway, is subjectivity: in fact in the beginning of the century Sigmund Freud's psychanalysis was born, and this raised culturatepeople's interest. Again, modernists and postmodernists don't agree one another and express two different opinions: modernists said a person was the result of his experiences and dreams and behaved accordingly, while postmodernists say that that people change their behaviour in each situation, so they are polivalent.
From this point two ways of writing develop: in a modernist work there are many references to the past and the story is almost absent and tells about the protagonist's routine, while a postmodernist nnovel, for example Coetzee's "Foe", focuses its attention on the novel itself, so you can call it a metaphysical novel.
As for knowledge the two thoughts are very different: modernists believed in universal and always-valid knowledge was possible , while postmodernists think knowledge isn't linear for it is made up up of many points of view, and that's why in postmodernist works there isn't an omniscient narrator.
In the twentieth century the concepts of space and time changed: in the modernist vision time isn't divided into past present and future as it is all in the characters' mind; postmodernists transformed alsothe concept of space: they think it isn't three-dimensional and believethe universe is based on non-Euclidean geometries. Also the concept of causality is seen from different points of view: linear in modernist vision and fragmentary in postmodernist.