Another error I see frequently in the written word is the confusion between thenand than.
Then has many uses:
"I went to the store, then I returned home." (an action that occurs after another one)
"The party starts at 11:00. I will see you then." (at a specific time)
"Take it, then." (equivalent of "in that case")
"She expected to be late, but then she didn't make an effort to be punctual." (as a qualifier)
Than is almost always used in the sense of comparing one statement or object to another:
She is older than I am. X is greater than Y. My car is less expensive than yours.