than / then
Comparison versus time or sequence.
Than marks a comparison: taller than her brother. Then marks time or sequence: first the soup, then the main course.
Examples
Use The second run was faster than the first.
Use We locked the gate, and then we left.
Avoid The second run was faster then the first. (Then is the error: faster sets up a comparison, and a comparison takes than. Write The second run was faster than the first.)