How to write email subject lines with AI + Prompts and Examples

The toughest part of email marketing? The subject line.

You put hours into creating valuable content, but this one line is what readers use to decide if it’s worth their time.

Copywriters say that headlines drive 80% of an ad’s success, and it’s no different for email subject lines.

At MoonDesk, we send a few dozen campaigns weekly. Through experience, we streamlined a process for creating stronger subject lines. I’ve noticed that a good subject line does two things exceptionally well: it sparks curiosity or clearly states the value. Ideally, it can do both.

10 effective approaches to write better subject lines

Here are my favorite 10 techniques to writing winning subject lines.

How to write subject lines with AI

I turned this table into a ChatGPT prompt that generates subject line ideas using each style. You can copy and paste it:

I will input a piece of content. Your output will be 2 options for each one of the following 10 approaches for a subject line following the pattern of the following examples. 

A good subject must do at least two things: Spark curiosity in the reader, or/and state the value they’ll get for reading. And you can do this in 8 different ways:

1. Tell them directly 
[Ebook] The Complete Email Marketing For Consultants For 2025
Our best performing ad formula
Watch this video to write better content.
Join the magic at B2B Forum

2. Arouse curiosity 
How one person changed my life…
70% of the decision is made before leads contact you
What Do Japanese Managers Have That American Managers Sometimes Lack?
One Ad Format That’s Crushing It Right Now – Try This

3. Empathize with a question
Are your email open rates killing your business?
Struggling to write good subject lines?

4. Announce a New
Finally, a simple email copywriting course you can take in a weekend
Announcing the Dream Career Formula - Fast Track Your Success

5. Start with a How-to Statement
How to use Webinars to grow your professional network… and revenue
[Webinar] How to Unlock SEO Growth in 30 Days
How content leaders create truly unique content

6. Use FOMO
Subscribe Today And Get A 90% Discount Your First Month
Only 24 Hours Left to Unlock This Bonus…

7. Use a list (steps, ways, reasons why)
30 social proof ideas to boost your authority
7 Steps to Success Coffee Shop Start-up Kit
12 Essential Steps In Building A Winning SEO Strategy

8. Own your experience
My favorite 7 subject line formulas
One email marketing mistake I’ll never repeat.
Can I share my first video with you?

9. Leverage trendy topics & current events
Marketing lessons from the Trump/Harris presidential campaign
How to use AI to write the perfect headline

10. Use Personalization
Congratulations, Hugo Sanchez
TED2014: Invitation to register for Hugo Sanchez
Hi Hugo Sanchez, please update your email preferences

For example, I ran the prompt for this article and this is what ChatGPT responded:

Here are three subject line ideas for each approach based on the content of your email:

1. Tell them directly
[Guide] Mastering Subject Lines: Our Top Strategies Revealed
MoonDesk’s Proven Formula for Writing Irresistible Subject Lines

2. Arouse curiosity
The Secret to Writing Subject Lines that Actually Get Opened…
Why This One Sentence Can Make or Break Your Email Success

3. Empathize with a question
Struggling with Subject Lines? You’re Not Alone!
Is Your Subject Line Holding Back Your Email Success?

4. Announce a New
Introducing: MoonDesk’s Ultimate Subject Line Guide!
Finally, a Simple Subject Line Guide That Works for You

5. Start with a How-to Statement
How to Write Subject Lines that Demand Attention
How to Make Subject Lines that Get Readers Clicking

6. Use FOMO
Last Chance: Discover Proven Subject Line Strategies!
Don’t Miss Out on Better Open Rates – Use These Tips Today

7. Use a list (steps, ways, reasons why)
8 Ways to Instantly Improve Your Subject Lines
8 Approaches to Subject Lines That Get Results

8. Own your experience
My Go-To Formula for Email Subject Line Success
The One Subject Line Trick I Wish I’d Known Sooner

9. Leverage trendy topics & current events
What ChatGPT Can Teach You About Subject Lines
How to Make Your Subject Lines Stand Out in 2024

10. Use Personalization
Hugo, here’s your guide to perfecting subject lines
Unlock better open rates, Hugo – start with your subject lines

These options will help spark curiosity or state value for readers, enhancing open rates. Let me know if you’d like further customization on any of these.

Using AI to improve your subject lines

Finally, remember to use AI carefully. Always check that what you put out there aligns with your voice and style… AI can become quite generic and “vanilla” over time. I typically turn to the AI prompt only when I’m struggling with an idea or to review a subject line I already have in mind.

For example, my initial idea for the subject line in this email was, “8 Subject Line Approaches I Wish I Knew Before.” I ran the following prompt on ChatGPT-4 preview:

Act as a professional copywriter specializing in email marketing, critique this subject line, and help me improve it: 8 Subject Line Approaches I Wish I Knew Before.

I always find these responses helpful. Here’s what the prompt returned (edited for conciseness):

Critique:

-Tense Consistency: The phrase "I wish I knew before" could be more impactful with a slight tweak, such as "I wish I'd known sooner" or "I wish I'd known before."
-Emotional Appeal: Phrases like "I wish I knew before" tap into the reader's fear of missing out (FOMO), which is good.
-Value Proposition: The subject line doesn't clearly state the benefit to the reader. Why should they care about these approaches?
-Brevity and Punchiness: The subject line is a bit long and could be tightened to make it more impactful.

Revised Subject Lines:

"8 Subject Line Approaches I Wish I'd Known Sooner"
"The 8 Secret Subject Line Strategies I Wish I Knew Before”
"Don't Miss These 8 Game-Changing Subject Line Approaches"
"8 Powerful Subject Line Techniques I Regret Not Knowing"
"Wish You Knew These 8 Subject Line Tips Earlier?"
"8 Subject Line Hacks Every Marketer Should Know"