Often we pronounce then and than similarly, but their meanings are quite different.  

Then is a reference to time, as in one event following another event.  For example, in "We went to the beach and then to the museum" then refers to the time or order in which the trip was made.  Then is also used with if, as in "If the temperature drops below freezing, then you must bring the plants and pets inside."

Than is used for comparisons.  For example, I would rather have dogs than cats as pets.  Than shows a preference for something.

Then:  If..,then...   Time

Than: comparison