What is the difference between Romantic and Modern era or periods?

The social-historical context for each era was considerably different. While the Romantic era writers were responding to the advent of democracy in the United States and Europe (and often advocating for the end of slavery), the Modern era posed an altogether different political reality.

Industrialization was fully underway for the Modernist writers and artists and a shifting world-order was quickly creating a new global-capitalist bureaucracy that brought colonialism to an end (or, arguably, eclipsed the colonial-imperialist programs of Western Europe). 

We can see the context for each period as integral to the content of the work associated with these eras. Romantic poems like Shelley's "Ozymandias" demonstrate a political outlook that was overtly condemning the rule of tyranny and authoritarianism. 

Shelley depicts a grand statue with a vaunting proclamation etched into its base:

"My name is Ozymandias, King of Kings; 

Look on my Works, ye Mighty, and despair!" 

Such ambition and power-hunger, Shelley's poem suggests, can only result in failure. And the poem's use of multiple narrators creates a subtle argument that this failure is guaranteed due to the broader democratic consciousness that has arisen to replace tyranny (and monarchy as well).

The statue, in the end, lies amid its own desolation. 

"Nothing beside remains. Round the decay 

Of that colossal Wreck, boundless and bare 

The lone and level sands stretch far away."

Compare this poetic sensibility with its commentary on the nature of power, ambition, and the collective voice to a poem like T.S. Eliot's "The Love Song of J. Alfred Prufrock."

In Eliot's poem the sense of the body politic has given way to a basic doubt about the integrity of the individual. In a world of rapid changes and remarkable mechanization, the individual naturally begins to ask what aspects of the self can be trusted to continue, what fate awaits the identity of an individual in a world of dynamic changes and growing commercialism. 

And indeed there will be time 

To wonder, “Do I dare?” and, “Do I dare?” 

Time to turn back and descend the stair, 

With a bald spot in the middle of my hair — 

(They will say: “How his hair is growing thin!”) 

My morning coat, my collar mounting firmly to the chin, 

My necktie rich and modest, but asserted by a simple pin — 

(They will say: “But how his arms and legs are thin!”) 

Do I dare 

Disturb the universe? 

In a minute there is time 

For decisions and revisions which a minute will reverse. 

The eras of Romanticism and Modernism can be distinguished along these lines, looking at the social context that informed the work. We can also look at the central subjects and metaphors that appear in these periods, noting that nature (or Nature, capitalized) is a recurrent motif in Romanticism and Freudian psychology is a motif in much of the most notable work associated with Modernism. 

A wholeness is found in nature for the Romantics. This wholeness is perhaps endowed with political meanings aligned with the ideas suggested above, but there is also a way that we can see the turn toward nature as a response to increasing urbanization in the 19th century.

As the cities grow, the poets find a metaphor for the essential human being in the countryside. Is this nostalgia? It might be. But it is also a telling element in the condition of the social landscape for writers in England, France and the United States. (The social landscape was mirrored by a changes in the actual landscape as roads were built, rail lines were placed for the first time..)

The Modern era saw the effects of this transition. Internal divisions between Id, Ego and Super-ego mimic a larger sense of schism as social divisions between classes became blurrier than they have been previously. Distances between cities shrunk with rail travel. During the Modern period airplanes appeared in the sky. The world that Modernist artists had been born into had been remade by the engines of industry and promised to continue to be remade. 

Thus we get poems like Ezra Pound's two-line poem, "In a Station of the Metro," depicting people as passive figments, parts of a commercial-industrial machine that has replaced nature and turned humans into ghosts of themselves. 

"The apparition of these faces in the crowd:

Petals on a wet, black bough."

No longer turning to nature as a metaphor for wholeness of the individual or for the power of collective and democratic government, Pound and other Modernists saw the power of subway lines and factory-line assembly as something of a symbol for the new social fabric, dominated by capital (as in capitalism).

We can see some of the reactionary qualities of Pound's vision in the difficulty of some of his work (i.e., Cantos) and the work of his contemporaries like Eliot and James Joyce. The challenging nature of the work was a comment on commercialism and an attempt to assert art against a rising tide of what we would today call "pop culture." 

The social dynamics of each of these periods continue to animate our contemporary world, to some extent at least, which partly explains why the art from these eras seems to speak so directly to us even today.