In Robert Frost's "The Mountain," what pivotal modernist and traditional themes are incorporated in the poem? How can the poem be used in the context of the North of Boston collection as a whole?

We don’t see many modernist themes in this poem; Frost leaned more toward the traditional ones. Modernist poets followed Ezra Pound’s command to “make it new,” but in Frost’s poems, including “The Mountain,” we see no experimentation in form. Rather, we typically see an adherence to the blank verse format, which signifies traditionalism. “The Mountain” has a very regular rhythm and meter; you can see how the text of the poem forms a neat, thin rectangle running down the page, and it’s this regularity, this adherence to well-established forms and conventions of poetry, that marks “The Mountain” as more of a traditional poem, something that resembles, at a quick glance, a page by Shakespeare or Milton.
Modernist poetry appears to us as jagged, full of experimentation with language and meter, and touching on dark themes of dissonance and dissociation within the mind. It also tends to portray a sense of personal disillusionment and meaninglessness, which often makes it difficult to read. On the contrary, “The Mountain” is easy to read. It’s a story about two men talking; it’s easy to picture it, as Frost gives us the visual detail that helps us easily imagine the calm mountainside scenery in which the poem’s action takes place, and the allusions in the poem are clear or at least unobtrusive. Reading “The Mountain” is easy. Reading modernist poems (like T. S. Eliot’s “The Waste Land”) is hard.
Still, we think of Frost as a “concealed Modernist,” someone whose works do show glimmers of Modernist themes. In typical Modernist poems, we see metaphors and similes all over the place, as if these were the favorite strategies of Modernist poems. But in “The Mountain,” Frost includes some figurative language (“The mountain held the town as in a shadow,” “to see [the stream] steam in winter like an ox’s breath,”), only to challenge the very idea of figurative language itself:

"Warm in December, cold in June, you say?”
“I don’t suppose the water’s changed at all.You and I know enough to know it’s warm.Compared with cold, and cold compared with warm.But all the fun’s in how you say a thing.”

Depending on how you interpret the poem, we do see one major Modernist theme in “The Mountain,” which is the idea of insurmountable barriers that prevent people from connecting meaningfully with each other. If you interpret the mountain in the poem as a kind of barrier that the narrator struggles to overcome, if you interpret the “scattered farms” as a failure of the people to form a meaningful community, if you see the failure of the narrator to understand the oxcart man’s explanation of the stream as a kind of breakdown in communication, and if you see the poem’s ending as an abrupt failure for their conversation to come to a meaningful conclusion, then you’ll come away with a modernist reading of “The Mountain.”
So, although the poem doesn’t neatly fit in either the traditionalist or modernist category of poetry, it does fit with the other poems in Frost’s North of Boston collection. How? It’s rife with natural imagery and includes some Biblical allusions, and it portrays regular people speaking to each other conversationally, the rhythm of their speech meshing into the rhythm of the poem. Similar to the other characters whose conversations dominate other poems in the North of Boston collection, the narrator and the man with the oxcart act out a kind of dramatic dialogue, a conversation full of tension and drama. As readers, we sense the urgency of the conversation, knowing that the narrator is eager for information and that the man with the oxcart is eager to continue on his way. The whole North of Boston collection is known as Frost’s “book of people,” and in “The Mountain,” the two people—narrator and oxcart man—seem to come alive through their conversation.