The basic rule
Then relates to time, sequence, or consequence: "First we ate, then we left." "If it rains, then we'll stay in." Than is used only for comparisons: "She is taller than her brother." "I'd rather walk than drive."
The trick
Than has an A, like "compArison" — if you're comparing two things, use "than." Then has an E, like "nExt" or "whEn" — if you're talking about time or what happens next, use "then."
Examples
Then (time/sequence): "We finished dinner, then watched a movie." "Back then, things were different."
Than (comparison): "This book is better than the movie." "Nothing is more important than health."
Frequently asked questions
What's the quick trick to remember then vs. than?
"Than" has an A, like "comparison." "Then" has an E, like "when" — both about time.
"Rather X than Y" — is that then or than?
"Than" — it's a comparison ("I'd rather read than watch TV").
Can "then" start a sentence?
Yes: "Then we went home" is a common, correct construction showing sequence.
Is this mistake common even among native speakers?
Yes, it's one of the most common typos in English, largely because the words sound similar when spoken quickly.