The speaker
Not much is clear about the speaker's identity, though his reference to his "green age" implies that he is young. Throughout the poem, he takes on a passionate and urgent tone, both in awe of and disturbed by the power of time.
Time
As the eponymous "force," time has a life of its own in the poem. The speaker characterizes it as both creative and destructive, harboring both life and death simultaneously.
Water
Water is the most common natural element in the poem. Like time, it can both create, as in a bubbling river teeming with life, and destroy, as in the quicksand of the third stanza.