I don’t know why, but the misuse of then/than really bugs the crap out of me. When I used to proof-read my daughter’s essays, if she used then or than, 99% of the time she got it wrong. In fact, at one point I remember telling her if she thought she should use than, use then instead and vice versa.
Just recently I read an e-book – not one of the free ones, but one I paid for – and the author kept using then when she should have used than. It pulled me out of the story every time.
Then is used when indicating time, when you want to indicate that something happens after something else.
At that time: Come to the restaurant, I'll tell you then.
Immediately following: First I will get a buy a new dress, then I will get my hair done.
In addition: She wanted to have her cake and then eat it too.
As a consequence; therefore: Jane wants to go to Europe, then, she works hard to earn the money.
In that case: If you want dance, ask me then.
Used after but to qualify or balance a preceding statement: James was a failure, but then he never worked very hard.
Used in "if...then" statements: If you want to win the marathon, then you must train every day.
Than is a comparative term that’s used when you're making a comparison between two or more things. When you are talking about a noun (thing, person, place or concept) being more, less, better, cooler, dumber, etc. in relation to another noun, the word than is necessary.
Robins are bigger than sparrows.
Gertrude has more money in the bank than I do.
Measles are more common than the mumps.
If you’re still a little unsure of whether then or than is the appropriate word to use, test your usage with a little substitution:
1. If you write the word next instead of then, will the sentence still make sense?
I will go to the pharmacy next makes sense. Therefore, I will go to the pharmacy then is correct.
I like books better next movies makes no sense. Therefore, you should use I like books better than movies.
2. If you write the phrase in comparison to instead of than, will the sentence still make sense?
It costs more in comparison to a new car makes sense, which means It costs more than a new car is also correct.
You'll never guess what happened to me in comparison to does not make sense, which means you will want to say You'll never guess what happened to me then!
If you think you’ve got a handle on the whole then vs. than issue, try testing your knowledge HERE.