The marriage of true minds
-- Speaker (Line 1)

Importance: This phrase is important because it means something quite different than what contemporary readers usually assume. A "marriage," in this sense, does not necessarily have anything to do with the legal ceremony. It is instead any union of two disparate things – like the minds of true people.

Admit impediment
-- Speaker (Line 2)

Importance: This line is ambiguous in its meaning. To "admit impediment" could be meant in the sense of admitting a guest – a refusal to allow any impediments in to a true romance. It could also mean that those impediments exist, but the speaker is refusing to admit that they do – choosing to remain willfully blind to problems.

Love is not love / which alters when it alteration finds
-- Speaker (Lines 3 – 4)

Importance: This is one of the most quoted segments of the poem, expressing a sentiment that is universal across the centuries since it was written – the desire to be loved unchangingly...