Too often - far, far too often - students of English forget that what must be at the core of their essays are answers to the question HOW. Just how has a writer made a text effective - what methods have been used in order to create a style suited to its particular audience and purpose?
There are many methods writers can use to create effects on a reader. In a story, there is effective use of narrative voice, characterisation, dialogue, irony and description, for example; in poetry, there will be the creation of effective ideas and images through the use of, for example, simile and metaphor as well as the use of rhyme, rhythm, alliteration, assonance and irony; in a play you should look at stage directions, dialogue and especially dramatic irony. NEVER waste words explaining what a technical term 'means' just assume your teacher knows this and get on with telling what effect it creates and the reason intended.
For an explanation of the terms used above, see the relevant Englishbiz page.