The one salient feature of Robert Frost that makes his poetry Modern, and makes him a modern poet is that, his poetry has been endowed with the awareness of the problems of man living in the modern world dominated by Science and Technology. 

However, Frost's poetry portrays the disintegration of values in modern life and the disillusionment of the modern man in symbolical and metaphysical terms as much as the poetry of great, modern poets does, because most of his poems deal with persons suffering from loneliness and frustration, regrets and disillusionment which are known as modern disease. 
One can identify such themes of disillusionment and loneliness in "An old Man's Winter Night", the poem is about an old man who is lonely, and completely alienated from the society. The tiredness of the farmer due to over work in "Apple-Picking"and as a result of it his yielding to sleep:

For I have too much
Of apple-picking: I am overtired
Of great harvest I myself desired

(it is even in his nature poems, that Frost has commented on the misery of the modern man due to his going away from nature.) 

Frost's metaphysical treatment of the subject in some of his poems is also an evidence of his modernity. In "Mending Walls", Frost juxtaposes the two opposite aspects of the theme of the poem and then leaves it to the reader to draw his own conclusion. The conservative farmer says:

Good fences make good neighbour

and the modern radical farmer says:

Something there is that doesn't love a wall,

Frost uses pastoral technique only to evaluate and comment on the modern lifestyle. His pastoralism thus registers a protest against the disintegration of values in the modern society.

Another poetic technique adopted by Frost which makes him a modern poet is symbolism. "The Road Not Taken"symbolizes the universal problem of making a choice of invisible barriers built up in the minds of the people which alienate them from one another mentally and emotionally thought they live together or as neighbors in the society. Similarly the Birch trees in "Birches" symbolize man's desire to seek escape from the harsh suffering man to undergo in this world.

 

Step-by-step explanation

Frost uses pastoral technique to comment on the human issue of modern world in his realistic treatment of Nature, his employment of symbolic and metaphysical techniques and the projection of the awareness of human problems of the modern society in his poetry justly entitle him to be looked up to as a modern poet.