Short answers, pulled from the story.
The written tradition of classical music began in the year 800 when a monk at the Abbey of Saint Gall wrote down a melody that became the foundation of Western music. This earliest extant music manuscript marks the start of a tradition that has survived for over twelve centuries.
The Baroque period of classical music started in the year 1600 with the advent of complex tonal counterpoint and the basso continuo. This era saw the rise of the violin family of stringed instruments and the development of opera as a staged musical drama.
The Classical era of classical music began in the year 1750 and established many norms of composition and style still recognized today. This period featured the standardization of the orchestra and the development of the symphony, string quartet, and concerto as major musical forms.
The modernist era in classical music began in the year 1890 when many composers rejected traditional tonality and structure. This period saw composers like Igor Stravinsky and Arnold Schoenberg explore new ways of organizing harmonic and rhythmic aspects of music.
The postmodern and contemporary eras of classical music started in the year 1945 with a diversification of styles and global expansion of the repertoire. This phase has seen the rise of new movements such as minimalism and spectral music influenced by folk and non-Western traditions.