What's the subject of the poem?

It may be difficult for some of us to follow the topic of Marvell's poem because of the compression of the poet's language. Once the poem is mastered, it seems self-evident how the topic unfolds, but at first some may be confused. There are two linguistic threads to follow that can help lead way through the movement of the topic: (1) the pronouns and (2) the shifts between metaphor subjects, specifically "soul" and "world."
The first thread to follow is the pronouns used. The subject of the poem is the "dew." Though the "rose" enters immediately, the topic does not shift to the rose: there is no digression from the "dew." From line 4 to "And to the skies exhale it back again," the pronouns it, its, 'twas [it was] itself, all substitute for "dew" in "orient dew" of line 1, "See how the orient dew."
Marvel does shift what he compares the dew to, so the second thread to follow are the shifts from the description of the dew to the comparisons that follow. The first comparison is between the "orient dew" and the human "soul":
So the soul, that drop, that ray Of the clear fountain of eternal day,
From here until "Does, in its pure and circling thoughts, express," all the it, its pronouns substitute for "soul." This section is short and is followed by a comparison between the "round" dew and the "world," ending "It all about does upwards bend."
The last four lines, two couplets, return to describing the "dew" in "Such did the manna’s sacred dew distill" and ending with "Into the glories of th’ almighty sun." The poem begins with a direct mention of "dew" and ends with direct mention of "dew." [A couplet is two lines that have end rhymes: distill/chill and run/sun.]