Short Answer 
The speaker of ‘Sonnet 18’ compares his beloved to a summer’s day, noting that while summer is beautiful, it has flaws that make it fleeting. In contrast, he presents his beloved’s beauty as superior and enduring, emphasizing her stability and resilience against the transient nature of summer.
Step 1: Compare Beauty
The speaker of ‘Sonnet 18’ begins by comparing his beloved to a summer’s day. He highlights that while summer has its charms, there are aspects that make it less appealing. The beloved is portrayed as more lovely and temperate, suggesting an idealized beauty that surpasses the typical attributes of summer.
Step 2: Highlight Flaws of Summer
In this quatrain, the speaker acknowledges the flaws of summer days, which can be affected by ‘rough winds’ and have a ‘too short a date.’ This emphasizes the fleeting nature of summer, where beauty can diminish quickly due to changing weather conditions. In contrast, the beloved’s beauty is presented as more enduring and stable.
Step 3: Emphasize Enduring Beauty
The speaker argues that the beloved’s beauty is not only superior to summer but also more constant and resilient amidst adversities. By presenting her as steady and long-lasting, the speaker conveys that her allure withstands the test of time, making her a more reliable source of beauty than the temporary, unpredictable summer day.