The poem "I Hear America Singing" was written by Walt Whitman. It was published in 1867 in the book Leaves of Grass. An earlier version of the poem, with slight variations, appeared in the 1860 edition of Leaves of Grass labeled simply as "20" under the section titled "Chants Democratic." 
The poem appears as a single stanza. However, some lines are broken into two but there is no enjambment. Counting the broken up lines as one line each, it can be concluded it is composed of eleven lines. When read through, especially aloud, the poem actually reads like a list. It has no set rhyme scheme or meter as it is written in free verse.
I hear America singing, the varied carols I hear,
From the first line, the reader is made aware the speaker (henceforth referred to with male pronouns) will be speaking of the diversity in America. He does not mention America as a whole; rather he builds a whole out of parts. This is evidenced by the "varied carols" mentioned in the line.
Those of mechanics, each one singing his as it should be blithe and strong,
The second line immediately develops the idea formed by the reader in the first line. Here, the speaker is specifying the carol, or song, of a specific group of people. This is one of the variations of America singing. He describes their song as "blithe and strong." He means their song shows indifference, or a state of being unconcerned, while also demonstrating strength. He also mentions "it should be" like this, implying he believes it to reflect their profession and personality.
The carpenter singing his as he measures his plank or beam,
The reader is then told of the carol sung by another type of person, the carpenter. Instead of describing the actual singing, as done previously, the reader learns what the carpenter does as he sings. The line provides a small glimpse into the daily life of this person, adding more variety to the image of America being conjured by the speaker.
The mason singing his as he makes ready for work, or leaves off work,
The mason, someone who works with stone, appears joyous. That he will sing as he heads to work and leaves work insinuates he is in an upbeat mood. If not in a good mood, at least the resolve of the worker has not waned at the end of the day.
The boatman singing what belongs to him in his boat, the deckhand singing on the steamboat deck,
Each worker on the boat has his own domain. The boatman and deckhand sing songs respective to what is theirs.
The shoemaker singing as he sits on his bench, the hatter singing as he stands,
Now, the speaker is mentioning more than one person per line. It is as if he is trying to fit as great a variety of people as possible into a short space. It is lovely, however, to imagine all these people singing as they go about their day. In this particular line, "the hatter singing as he stands" is an interesting image and use of words. The reader can almost imagine a hat-stand from this line.
The wood-cutter’s song, the ploughboy’s on his way in the morning, or at noon intermission or at sundown,
All day long, the people are singing. Though this is directly stated after the ploughboy is brought up, it is likely true of all or most who have been introduced in the poem thus far. Perhaps the songs can be taken as an extension of the people’s moods and thoughts and that is why they are continuous.
The delicious singing of the mother, or of the young wife at work, or of the girl sewing or washing,
"The delicious singing of the mother" is a description that certainly stands out from the rest. It is something that is highly pleasant and speaks to the importance of the mother. Within the same line, other women are mentioned for the first time in the poem.
Each singing what belongs to him or her and to none else,
This line speaks to individuality. Though every person is part of a group, that group of a larger taskforce, and that of the nation, every song has something unique. Each person sings "what belongs to him or her and to none else." Though there is unity throughout, that does not take away the uniqueness of the individual.
The day what belongs to the day—at night the party of young fellows, robust, friendly,
Whatever the carol may be for the day, it is different for the night. Just as each person sings his or her own way, daytime and nighttime have distinct songs. The daytime song varies greatly while the nighttime song, generally, is lively and warm. At is alive with the animation of "young fellows."
Singing with open mouths their strong melodious songs.
These people are not ashamed and do not keep quiet. The songs are loud and harmonious. These are common people who appear in the poem. They are the workers of America who help keep it up and running. These are people who endure physical labor. These are people who are united and proud. This is reflected in the cheery tone of the poem.
As previously mentioned, "I Hear America Singing" reads like a list. Of course, it has more musicality than a typical list but it does catalog an array of people. The speaker discusses the carols America sings by detailing the songs of groups of individuals. The reader is shown how regular, working people go about their day as they sing. Both high spirits and strength appear in the poem through the descriptions, creating a positive image of the people mentioned.
The theme of this poem is work. It celebrates America in a very specific way. The portrayal of the average citizens is positive and energetic, something that is uplifting. The people do not seem to complain of their daily lives and tasks. Instead, they appear to enjoy and appreciate them. In this regard, a theme of pride also surfaces.