Than is used for a comparison. This book is longer than that one. It is cloudier today than it was yesterday.
Then is used for sequences or consequences. We are familiar with if/then simple sentences.
The problem seems to come when it isn’t a straight if/then sentence then the error occurs more often. In reading the preceding sentence it becomes apparent, hopefully, that it is an if/then. The word if isn’t used but if we dissect it a little we can see the if/then structure.
The problem seems to come when it isn’t a straight if/then sentence then the error occurs more often.
When Bobby gets here then we will go.
Being able to replace when with if shows this is and if/then type of sentence. This isn’t a comparison it’s a cause-effect, a consequence.
These are simple ways to help decide which to use; then or than.