Full answer
A cover letter has one job: persuade a hiring manager to open your resume. The most common failure is a letter that restates the resume rather than adding new information. Your cover letter should answer the question the resume cannot: why this role, at this company, right now.
The opening line should not be "I am writing to apply for..." — that wastes the most-read sentence in the letter. Instead, lead with a specific observation about the company or a statement of your most relevant qualification. Something like "Three years running customer operations at a 50-person SaaS startup taught me that the gap between good support and great retention is almost entirely about response speed" is immediately more compelling than any generic opener.
The body (two short paragraphs) should draw a direct line between your past achievements and the role's requirements. Quantify wherever possible: reduced churn by 18%, managed a $2M budget, cut deployment time from 4 hours to 45 minutes. Specific numbers are memorable and credible in a way that adjectives like "detail-oriented" and "self-motivated" never are.
The closing paragraph should name one specific thing about the company — a product, a value, a recent initiative — and explain why it resonates with you. Generic closings ("I am excited about this opportunity") signal a form letter. End with a direct call to action: "I would welcome a 20-minute conversation to discuss how my background maps to your Q3 hiring goals."