What is the main argument of the poem "Sonnet 73"? With which three tools does the author prove this argument?

The main argument in William Shakespeare's "Sonnet 73" is that passion grows with age. The speaker describes this passion as a "glowing . . . fire." The speaker tell his beloved that passion increases because of the knowledge that death, which is presented as "black night," is drawing near.
Shakespeare's tools in the sonnet are meter, rhyme, and metaphors. The meter is iambic pentameter. The rhyme scheme is A-B-A-B, C-D-C-D, E-F-E-F. He uses three conceits, or extended metaphors, to establish comparisons to the speaker's age: the season, winter; the time of day, twilight; and the fire, ashes. The season or "time of year" is shown by the scarcity of leaves on the trees. The day fades into twilight "after sunset . . . in the west." The fire still glows against the background of "the ashes of . . . youth."
All three conceits connect with the finality of death, which "seals up" everything as a final repose.

The main argument of Sonnet 73 is that, in loving someone whom he knows to be in the "twilight" of his life, and who is very obviously mortal, the beloved demonstrates that his love is "more strong." It is representative of a stronger and more devoted love "to love that well which thou must leave ere long."
Shakespeare uses a number of tools to convey the message in this poem. A symbolic field of autumn pervades the poem, with an extended metaphor used to indicate that the poet's appearance reflects "that time of year" when "yellow leaves, or few, or none" can be seen "upon those boughs which shake against the cold." The poet characterizes himself as a tree in autumn, with its leaves fallen off. Next, another extended metaphor compares the poet to "the twilight of such day / as after sunset fadeth in the West." The shadow of death is alluded to when the speaker discusses "black night," "Death's second self," which takes away all who have lived. Finally, a metaphor pertaining to fire is used to describe the poet, who is now "glowing" "as on the ashes of his youth." Where once a fire burned high in the poet, his youth has now ebbed away such that there is little left to fuel the fire, leaving it only "glowing" and "ashes."
It is clear, then, the poet says, that his beloved knows the poet is getting old. He cannot avoid seeing this to be true: "this thou perceiv'st." As such, the beloved's love must be all the stronger, because he still loves the poet, despite the fact that their love cannot last much longer.