Though the word than should technically rhyme with man, many of us are guilty of pronouncing it exactly the same as the word then. This, combined with the fact that then and than only differ by a single letter, helps explain why people have so much trouble choosing when to use the correct one.
When to Use Then
If you’re talking about time, you probably want then.
As Merriam-Webster explains, common phrases involving time use then, too—like since then, back then, and every now and then. So do conditional statements with if, such as:
If I take a nap, then I’ll have enough energy to clean the kitchen.
When to Use Than
Than, on the other hand, usually involves a comparison: less than, more than, bigger than, rather than, other than, etc. Choosing than might seem obvious when it’s immediately preceded by a comparative term, but it can be a little less clear when there’s a phrase between them.
Taking a nap is more of a priority than cleaning the kitchen.
I’d rather take a nap than clean the kitchen.
This can get especially confusing if you’re starting a sentence with the phrase No sooner had or No sooner did. Yes, you’re talking about time—but since sooner is functioning as a comparative term, you still want than.
No sooner had I awoken from my nap than my roommate texted to ask if I had cleaned the kitchen.
And if someone ever points out a then-vs.-than mistake of yours, you can always just claim it was an intentional nod to Middle English. Back in the Middle Ages, the two terms were often used interchangeably.