Structures

Before we analyze the structures of the two pieces,  we should first analyze what the structure of a piece of music even is. A songs structure is how it's organized. An example of the structure of a song is intro - verse -hook - chorus - verse - hook- chorus -bridge - chorus -coda. (this is the structure of most popular music today). To further analyze what a song structures are, these are the definitions of the parts of the structures:

Chorus: The chorus is a lyric-music combination that is repeated with little or no change throughout the song. Usually, the chorus lyrics include the song title and gives the listener a general idea about the song topic. The chorus music includes the most catchy melody in the song...The chorus is generally the part that your listener will remember from your song.

Verse: The verse has the second melody of the song, this melody hardly changes throughout the song. What changes is actually the verse lyrics; as the verses usually give details about the song topic.

Bridge: The third melody and the one that is -in most song structures- appearing only once. It is better if the listener gets a "tiny" surprise in the bridge; that can be a change in the chord structure accompanied by a smooth shift to another scale...etc. However, the bridge melody always ends by shifting back to the original mood and repeating the chorus all over again. As for the bridge's lyrics they usually represent a conclusion or a flash back to the whole song, this adds to the surprise. 

Coda: The outro of a song- the ending.

The structure of Gershwin's Overture is ternary form. Ternary form is a three-part music form-usually schematized as A-B-A. What this means is the first and third part (the A's) are musically identical, and if not very similar. The B section provides a contrast from the two A sections. The B section is generally in a contrasting, but closely related key, usually the dominant key of the first section's tonic (a perfect fifth above) or the relative or parallel minor or major, less often the sub-dominant key (a perfect fifth below).

The first part (moderato e molto ritmato) is preceded by a (forte) introduction featuring some of the thematic material. Then comes a three-part contrapuntal episode leading to a second theme. This first part finishes with a recurrence of the first theme combined with fragments of the second.

A solo clarinet cadenza leads to a middle part, which is in a plaintive mood. It is a gradually developing canon in a polytonal (which means  several tones together) manner. This part concludes with a climax based on an ostinato of the theme in the canon, after which a sudden change in tempo brings us back to the rumba dance rhythms.

The finale is a development of the preceding material in a stretto-like manner. This leads us back once again to the main theme.

The conclusion of the work is a coda featuring the Cuban instruments of percussion.

The form for Piazzolla's Invierno Porteno is not in a certain form. This is the structure of the Tango from beginning to end:

Slow intro Theme A (minor key)
Fast - theme A’
Transition
Slow tempo- Theme B
Fast Tempo- New Material (pentatonic –open 5ths) Theme C
Slow tempo (cello) Theme D
Theme D passed to violin
Fast Tempo Theme D’
E theme passed to piano
Slow Theme D
Slow theme new material “E” –passed from strings to piano.

The form of Piazzolla' piece is built among the tempos of the piece. All Tangos follow the pattern of a slow intro, then the tempo speeds up and the pattern continues until the slow outro. The slow tempo sections are in 3/4 time, while the faster tempos are in 4/4 time. (the 4/4 time signature gives it more of a moving forward pace).