Defining key aspects of the modern--can't be done simply. But why not try? Here's one. The modern poem isn't about expression or expressiveness, something the poet has urgently wanted to say. It's primarily neither topical nor personal in the accepted 20th-century sense of the person who has things "inside" that must be said, written, conveyed. The poem isn't telling you you should or must know something. It doesn't cover or fill a gap, a need, a want. The poem is merely (oh that huge "merely" - but I don't mean it trivially) a means of keeping a reader from going from it, a detention, a planning to stay, and then--in it--is a remnant of the poet, all we know of him or her at that moment, then (now, the time of coming upon the words) and here (in the poem itself, making an inside that's nowhere else but where it is).
To the extent that the above definition is apt and useful, then the modern verse mode derives largely from Emily Dickinson, who in more than half her poems makes the point I've made above the matter of the poem.
And Cid Corman, not otherwise deemed Dickinsonian, is surely getting at this in this poem:
It isnt for want
of something to say--
something to tell you--
something you should know--
but to detain you--
keep you from going--
feeling myself here
as long as you are--
as long as you are.
And here is a recording of Cid Corman reading that poem (and another short one after it).