Why do people often confuse than and then in writing?

Writers often confuse the words than and then because they sound alike when spoken. Remember that than is used to make comparisons and then is used to indicate a point in time.

Incorrect: You reacted more calmly then I would have.
Correct: You reacted more calmly than I would have.
Incorrect: They changed clothes and than they went swimming.
Correct: They changed clothes and then they went swimming.