By far, the biggest mistake writers make when writing subject lines is making them “too long.”
If a subject line gets cut off before “the hook” is revealed, the chances of a reader clicking is very low. Why? Because they don't have a reason to! They don't know what you're promising inside—which means they don't know what to expect (or why it's worth their precious attention).
Which leads us to Rule #1.
A good rule of thumb is your subject line should not exceed 15 words. Ever.
Less is always more here. If you can say it in 15 words, try to say it in 10. If you can say it in 10 words, try to say it in 7. If you can say it in 7, try to say it in 5 words. However, “less words” doesn't inherently mean the subject line is “better.” The key is to give the reader a “hook” that makes them want to click and open the email—but to say that “hook” in as few words as possible WITHOUT LOSING THE MEANING.
For example:
Take the subject line: “The 1 thing you need to do to strengthen the microbiome in your gut,” shortened to: “The 1 thing you need to do.” Yes, it’s shortened (which is good), but the whole idea in the “hook” is lost (which is bad).
A better rewrite would be something that says the same thing, but in less words, like: “Want to strengthen your gut microbiome? Try this 1 thing.”
Same idea, but the first subject line is 16 words.
And the second subject line is 10 words.
This comes down to grammar & punctuation more than anything else.
For example:
In general, the second subject line here will see better open rates.
Why?
Because your inbox is personal. And personal feels relevant (interesting to open) for the reader. Think about it. If you’re writing an email to a friend, are you going to capitalize every word in the subject line?