Language is the words that are used and what they do. For example, nouns, which name places, things or people, and verbs, which describe actions. 

Form is the type of text or poem. For example, the poem could be a sonnet - a poem of fourteen lines in iambic pentameter. 

Structure is how the poem is put together. Think about the sentence length, the structure of stanzas or the order in which the story/argument is structured as it tells a story.