What is sonata form, and how can I identify it when analyzing a piece of music?

Sonata form is often found in music of the Classical era, for example in the string quartets or symphonic movements by composers such as Mozart, Haydn, and Beethoven. Sonata form is essentially a three-part structure: the three main sections are the exposition, the development, and the recapitulation. In the exposition the two main theme groups are introduced (which can be labelled as the primary and the secondary material). The primary material will be in the home key (I) and the secondary material will be in a different key (usually the dominant (V) or in a minor key, the relative major (III)). These two themes will be separated by a transition passage during which the music modulates. In the development, ideas from the exposition may be heard again with a varied presentation, moving through a variety of keys. In the recapitulation, the opening material returns. Crucially, in this section the secondary material will return in the tonic rather than the key it was originally heard in. This may then be followed by a coda.As far as analysis goes, a useful approach is to first identify the main sections (the end of the exposition will usually be marked by a repeat marking, and the recapitulation will see an evident return of the opening material). Then, comparison of the exposition and recapitulation should allow you to determine which material returns in the same key, and which returns differently (the primary and secondary material). The transition is likely to begin the same in both exposition and recapitulation but will then differ at the moment when modulation is avoided (try and look out for accidentals and when they do and don’t appear). Within the development, try to identify whether the same ideas have been seen before, and if so, from where. Finally, consider the ending: is this a new presentation of material that might be considered a coda?