To answer your question, it is best to look at the definition of each terms to notice the differences so we can distinguish each from the other.
Dr. L. Kip Wheeler states:
FORM: The "shape" ...of a particular poem. In most poems (like sonnets), the form consists of a set number of lines, a set rhyme scheme, and a set meter for each line. In concrete poetry, the form of a poem may reflect the theme, topic, or idea of the words in the actual shape of the text on a piece of paper.
So, the form of a poem, for instance, talks about how it is put together and how it will look. An Elizabethan sonnet will have fourteen lines: made up of three quatrains (three four-line stanzas) and ending with rhyming couplet (two lines that rhyme at the end of each line); its rhyme scheme is abab cdcd efef gg. The Italian sonnet will have fourteen lines, but the form is different.
For a concrete example of form with a concrete poem, for Easter, a poem might be shaped like a bunny; for Christmas, it might be shaped like an angel. If the poem is about "Angry Birds," it could be shaped like the head of an "angry bird." Its shape is its form.
In terms of structure, this is must more substantial. According to Lilia Melani, of Brooklyn College, structure is:
...[the] framework of a work of literature; the organization or over-all design of a work. The structure of a play may fall into logical divisions and also a mechanical division of acts and scenes. Groups of stories may be set in a larger structure or frame, like The Canterbury Tales, The Decameron, orThe Arabian Tales.
For a more concrete explanation, now we look to the underlying structure that guides the development of the plot. Though it may seem secondary, it is of primary importance in shaping the piece of literature. Look at the examples above.
Chaucer's The Canterbury Tales is a collection of stories; it's structured around a holy pilgrimage to Thomas Becket's place of martyrdom at the Canterbury Cathedral. A pilgrimage is a holy journey to a Christian landmark where people of all socioeconomic levels would join together to travel to a landmark, often a shrine, to praise God. Because the structure of the tale is based on the pilgrimage, Chaucer can bring together the poor, rich, hardworking, honest, dishonest, hypocritical, etc., in one story: all people going on a pilgrimage.
The structure of the Arabian Knights or A Thousand and One Nights is structured around the woman who is trying to save her life, avoiding her execution.To live, she must tell a new story every night: and does so for one thousand nights to keep him entertained. As long as she can do so, she will be spared. The structure of the story is that the woman must tell a new tale each night to save her life. This is structure that pulls all the different stories together thats she tells.
Language can mean many things:
Language can be classified in a number of ways.
There is denotative and connotative language: what is a dictionary definition or what is associated with a word that gives it a meaning other than the dictionary's definition. This can also describe figurative language which is more poetic and less realistic. Similes and metaphors are figurative language. Ex. "You are the sunshine of my life." This is a metaphor—the topic of the song will not give someone sunburn: it's not literal.
There is also abstract or concrete language, and literal or figurative.
Language is the wording you choose to use to write your piece.