Introductions should –
put the essay title into your own words
state the poem and the extract you are going to write on
give a brief summary of the plot of the poem / extract
identify one or two main topics or themes in the poem / extract - these will be the topics you will structure your essay around
list a few of the main literary / poetic techniques the poem / extract uses
put the poem / extract into context - if you are writing on an extract briefly summerise the other parts of the poem you are not writing about, put the poem into the context of Birthday Letters be very concise, only refer to what is relevant.
There is a lot to pack into this introduction. You might want to leave it till after you have written the main 6 - 8 analytical paragraphs. But you must be concise