There are many types of song structures, ranging from a 1 chord funk groove to a highly complex jazz arangement involving many different keys, to standard 12 bar blues progressions etc.
It is important to be able to identify the structure of a song in order to grasp the logic behind the chord progression and melody line. Most songs are written with two separate sections, the " A " section, being the head of the song or theme, and a " B " section or bridge, to provide a contrast to the song by providing another melody to add excitement and interest to the piece. The most common structure is an A,A,B,A formula, however there are many different possibilities.
The next thing to familiarise yourself with, are the keys within the song. Some will stay completely within one key, but more often there will be a few twists and turns to add melodic interest by modulating to at least one other key.
These key centres can usually be identified by a 2, 5, 1 ( II, V, I ) progression. The II chord being the second chord in the Major Chord scale (a minor chord) progressing to the dominant 7th or V7 chord then to the I chord or Tonic of the key. e.g. the II, V, I progression in the key of " G " major would be: A minor to D7 to G major or Am7 to D9 to G Maj7 or any other combination of minor to dominant to major.
In a minor key the II V I progression chords would usually be a minor7b5 ( II chord) to dominant7b9 ( V chord ) to the Tonic minor chord. It is very important to be able to recognise II V & II V I progressions in both Major and Minor keys. Especially if you are improvising over the chord progression.
By identifying the main key centres you will know what scale choices are available. Even just locating the dominant chords can be useful, as they can indicate a temporary key change or lead back to the tonic chord. Scalewise progressions are also very strong, not only using 2,5,1 progressions but also 1, 6, 2, 5 chord movements.
The next thing, is to look at the chords themselves. Chords in a progression tend to move freely to chords built on their 4th or 5th, often referred to as the Cycle of 5ths . Blues progressions especially, move around this Cycle in 4ths.
Chromatic progressions are also quite common, chords tend to flow chromatically (particularly in a downward direction) very smoothly. eg. Em7 to Ebm7 to Dm7 to Dbm7
Symmetric progressions also work well, once a pattern has been established, a chord progression will flow freely along that repeated pattern. example; C to Eb to Gb to A is a symmetrical pattern, moving in minor 3rds each time.
Relative major to to relative minor chord changes introduce another dimension in chord progressions. Each chord has a relative major or minor counterpart. These chords are very closely connected, (eg. a " C " major 6th chord contains the same notes as an " A " minor 7th it's relative minor chord.)
Chord TYPE changes. Chords can easily be changed from major to minor etc eg C Maj7 to F Maj7 to Fm7 to Bb7 to Eb Maj7. Note that a " CHORD TYPE " change often marks the beginning of a new key area. Another good example of chord type change is the song " Something " in the George Harrison version the opening chords are C to C Maj7 to C7 .
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