Final answer:
The first two lines of the quatrain suggest the sun can shine too intensely or be hidden by clouds, while the last two lines reflect on the transient nature of beauty.
Step-by-step explanation:
The best paraphrase of the first two lines of the second quatrain of Shakespeare's 'Sonnet 18' might read: Sometimes the sun (shines) too brightly, and often its golden appearance is clouded over. The last two lines can be paraphrased as: Everything beautiful eventually fades away, either by chance or through the natural process of change that goes unchecked.