Which is correct?
Chocolate ice cream is better than vanilla.
Chocolate ice cream is better then vanilla.
The correct choice is the first sentence using than.
Than is a subordinating conjunction. It is used in comparisons, as in the example sentence above in which two flavors of ice cream are being compared.
Then is considered a conjunctive adverb. It is used in reference to time, indicating that one action follows another.
We spent the morning sightseeing; then we stopped for lunch at a local restaurant.
Then and than are often pronounced as if they were the same word, which may explain why they are frequently confused in writing.