Firstly, it is essential to know the difference between the three. The common problem is differentiating form and strucutre. The best way to think about this is that structure is much more simple! It is simply the way the poem is technically aranged (stanzas, rhyme, style). With Form it is best to think about the hollistic, or overall, appearence of the poem to get at the way that the structure is used (tone, voice, tense). There is overlap here. Language is usually less difficult, beceause you just have to break down the words in the poem. What word choice has been used, what devices (alliteration, enjambment, oxy-morons). 

It is often best in this scenario to conclude by trying to bring all three aspects together - what is the poet's overarching message?