In the third quatrain (lines 9–12), the speaker makes a daring statement to his beloved. What does he claim will never happen?"Shall I compare...

Shakespeare's Sonnet 18 is ultimately about the poet's
belief that his "eternal lines to time" (this poem) will allow the person he is speaking
to to live forever.  He claims in the final couplet that as long as people are reading
this poem "so long lives this, and this gives life to thee."  The "this" that he is
referring to is the actual words/language of the poem. 

Now
that you know what the poem is actually about, the third quatrain actually makes a bit
more sense in the context of the conclusion.  The poem opens with two quatrains of
comparisons explaining how his love is better than a summer's day. 
The third quatrain claims that the beauty of the beloved will never fade (like real
summer does), and Death will never have a chance to brag that the beloved is with
him--not because the beloved will never die, but because even though the person will
physically die, they will never be dead and gone because they will live on through this
poem.  As it turns out--Shakespeare was right!  Here we are nearly 400 years later,
taking about the beauty and life of his beloved.