Milton's contribution can be identified two ways. The first is his contribution to the corpus of English literary works, such as three of his greatest: Paradise Lost, Lycidas, Samson Agonistes. Additionally, Milton broke with his contemporaries, who were largely following what we call the metaphysical tradition, and turned back to the Renaissance and to Greek and Latin poets. Thus it is he made a contribution to poetic and tragic genre by reviving the greatest earliest traditions, for instance, Samson Agonistes is styled as a classic Greek tragedy.
This introduces the other way his contribution can be measured, that is his contribution to the English language. While acknowledging his linguistic debt to Shakespeare in "On Shakespeare," introducing his 1632 folio, Milton followed in Shakespeare's footsteps by expanding the English language for poetic and dramatic effect. Gavin Alexander of Christ's College, Oxford University, expresses this aspect of Milton's contribution like this:
... to remake words to bear new meanings, to create a word or phrase where the language offers none, to stretch imagery and syntax in the effort to represent emotion and thought. This is why Milton stands next to Shakespeare in the English poetic tradition
He too, like Shakespeare, altered syntax and grammar to most effectively express an idea, or to most dramatically express it. He too relied on extended simile and metaphor, like in epic similes. He too coined words from Latin borrowed words. Some of these dropped out of the lexicon with Milton, but many coined words are still active contributors to the English lexicon. A few examples of these are: embellishing, besotted, unadventurous, reforming, slow-motion, chastening, unintended, defensively, padlock, disregard, attacks, enjoyable, awe-struck.
A couple of examples of these points from Milton's greatest works follow.
Paradise Lost: Word coining: bliss:Restore us, and regain the blissful Seat,
Syntax:Him the Almighty PowerHurld headlong
Samson Agonistes:Extended metaphor underlying tragedy:... and find himEyeless in Gaza at the Mill with slaves,Himself in bonds under Philistian yoke;
Lycidas:Classical Greek allusion:Begin then, Sisters of the sacred well,That from beneath the seat of Jove doth spring,
Syntax:Compels me to disturb your season due: