Out of your whole life give but one moment!
All of your life that has gone before,
All to come after it, – so you ignore,
So you make perfect the present, – condense,
The opening sentence is short and abrupt, with an exclamation mark to emphasise the importance of the imperative; it is what the poem will be about, the focus of a single moment of perfect love. 
 In terms of construction, Browning begins with a preposition and reverses the usual syntax of the two clauses. If this were re-worded colloquially it would read “Just give me a moment of your life”. The device  anastrophe  surprises the reader and draws them in.
Now