The difference between "then" and "than" is fairly simple, but many people, including experienced English speakers, have difficulty with these two words. In fact, the two words are totally different parts of speech, used in entirely different ways; the confusion is probably linked to the fact that they sound very similar in spoken English, making it difficult to tell which word is being used. Knowing the difference between them will greatly improve the readability and quality of an individual's written English.
The word "than" is a conjunction, which means that it links two clauses. This word is further classified as a subordinating conjunction, which means that it establishes some sort of relationship between two clauses — in this case, the relationship is comparative. For example, someone could say that "this apple is bigger than the one I had last night," or "Was the sequel better or worse than the first movie?" Any time a person is comparing two things, "than" is the appropriate word to use.
On the other hand, "then" is an adverb, meaning that it modifies a part of speech or a clause. Specifically, it is used as a conjunctive adverb to join two clauses that are separated by time; "then" acts like a unit of time in a sentence, telling when something happened (or is going to happen). For example, one could say "He went to the store, and then stopped by the park," or "Please do your homework, and then you may watch television." In both of these sentences, the word could be replaced by "after that," and the sentences would make sense.
When someone is looking at a sentence and trying to figure out which word is appropriate, he should think about what he is trying to convey with the sentence. If he is comparing something, he will want to use "than," while if he is discussing the time at which something occurred, "then" is the most appropriate choice. One might use the simple trick of remembering that "then" rhymes with "when," or use the "after that" trick, by replacing the word in question with "after that" and seeing if the sentence still makes sense. Using examples from above, "The apple is bigger after that the one I had last night" is rather nonsensical, while "Please do your homework, and after that you may watch television" works as a sentence.
The difference between these two words is far from subtle, and mastering it is crucial for people who want to be communicate clearly, effectively, and professionally. Fortunately, unlike the difference between "effect" and "affect," the difference between "then" and "than" is quite easy to remember, making it easy to use these words correctly.