I am not going to write the whole 400 words for you, but here are some ideas to get you started.
Although Hopkins was not recognised as a major poet in his lifetime, he is now regarded as one of the key "modern" poets. His major tour de force which exemplifies what he brought to poetry is "The Wreck of the Deutschland", which narrates the death of 5 nuns in a shipwreck on the coast of England who had been exiled from Germany. In this poem, Hopkins introduced his revolutionary sprung rhythm. This rhythm, in contrast with traditional rhythm, which is based on the alternation of stressed and unstressed syllables, is based on stressed syllables alone.
Along with this new meter, this poem introduces other aspects of Hopkins' art, in particular his use of diction. He uses unusual compound words, terms borrowed from dialect and coined phrases. To this he adds internal rhyme, elliptical meaning, half rhyme and compression along with assonance, alliteration and metaphor.
Apart from these stylistic innovations, this poem also introduced Hopkins' revolutionary philosophy of poetry. This poem (and his later, shorter, works) reflect Hopkins' belief that man was made to praise God and we can see nature praising God. Also, it draws heavily on Jesuit practices of meditation and spiritual self-examination.