My husband has a Ph.D., and he used to get then and than confused all of the time. Even in published works I often find these two words used interchangeably. I think the confusion has to do with their similar pronunciation. Unfortunately, their definitions are also alike.
According to Google then is defined as:
1. at that time; at the time in question.
2. after that; next; afterward.
Google defines than as:
1. introducing the second element in a comparison.
“he was much smaller than his son”
2. used in expressions introducing an exception or contrast.
“he claims not to own anything other than his home”
However, the easiest way, for at least me, to remember the difference is to associate one with time and one with comparisons. Some folks like to use a trick called mnemonics that implements letters, patterns, words, or associations to help them remember. I know I’ve read about this trick before, but I can’t recall where; however, in searching the web for then/than, I came across Grammar Girl’s blog that offers the same suggestion. [Grammar Girl Quick and Dirty Tips] Time has an e in it and so does then. Comparison has an a and so does than.
If you are comparing something use than.
Do you like tea more than coffee? (comparing tea to coffee)
Cats are generally smaller than dogs. (comparison)
He was growing faster than a weed. (comparing his growth to a weed)
If you refer to time, use then, or when using if/then statements.
If you go to the store, then pick up a gallon of milk. (If/then statement)
She turned the knob, and then walked through the doorway. (time/progression)
Tell me then, do you often quote Shakespeare? (tell me now/tell me next – a reference to time)
Hopefully, that helps clear up any confusion regarding these two words, but if you need a little practice, then take the ten question quiz below.
Thanks for reading!