Grammar : Then and than

These are minor mistakes I find constantly in assignments.  Unfortunately, small errors in word use make your work look bad and reduce grades.  Moreover, these are mistakes that are not helped by the use of a spell check programme.  When you use spell check, you have a false sense of security.  I'm not advocating not using spell check; clearly, this course of action would be a mistake.  I've also seen the result of not using spell check, compositions peppered by mistakes and students who apologetically say they didn't "have the time".  No instructor will accept this excuse for not doing better.

So, if spell check won't fix your mistakes, what do you do?  I strongly urge the use of a book such as Advanced Grammar in Use by Martin Hewings (Cambridge Press) which has a disc with good grammar exercises.  Another book, A Canadian Writer's Reference by Diana Hacker is also helpful with another exercise book.  Of course, ultimately I recommend the use of a really good dictionary, "The Oxford Concise" or another large Oxford to help you out.  There are other dictionaries but I recommend the Oxford as time tested and excellent.

Then/Than
Than is a conjunction.  Conjunctions, as Robert Allen says, are "joining words" (Allen, How to Write Better English, 96.  Another way of describing them is as "linking words."  Conjunctions are viewed by many as "sentence connectors".  Many conjunctions join the main clause (the main part of the sentence with the subject & verb) to the rest of the sentence.  In the case of than, the Oxford Dictionary of English states there are three uses of this word:
1. As a comparative - She was much taller than her mother.  or "Jack doesn't know any more than I do." (Oxford Dictionary of English, 1826).  In this instance, the use of "than" is definitely as a conjunction, a subordinate conjunction in the subordinate clause.
2. "Used in expressions of exception or contrast.  He claims not to own anything other than his home."Note: here than is a preposition. (Ibid.)
 3. Indicating one thing happening right after another.  Scarcely was the word completed than it was abandoned. (Ibid.)

Then
When you look at the Oxford Dictionary, it tells us that than and then used to be the same word.  If you look at the third meaning, you can see how this might work.  However, English changes and evolves.  Then is now a completely different word, an adverb connected to time.  We must work in modern, not archaic, English.  An adverb, as you are aware, modifies or describes the action of a verb.
For example:
She bought a pair of red and gold shoes at Lewis' that afternoon then wore them later that night.

One thing happens then another follows it.  This is the nature of then, time.