"Shall I compare thee to a summer's day?..."
See in text (Sonnets 11–21)
The speaker compares his addressee (likely a young, male aristocrat) to a summer's day. However, the comparison is backwards. Summer is measured against the young man and found lacking: he is "more lovely and more temperate," given that summer is short and can be brutally hot. Though the young man's beauty and youth cannot last forever, the speaker concludes that both can be immortalized through poetry.
"Let me not to the marriage of true minds
See in text (Sonnets 111–120)
"True minds" refers to fidelity as opposed to authenticity. In Shakespeare's time, the word "true" could mean constant or faithful. The speaker, who is frustrated by his lover's inconstancy, insists that "love is not love" if it extinguishes or alters in the face of change.