THEN       (thěn)

adverb. 

At that time: I was still in school then. Come at noon; I'll be ready then.

Next in time, space, or order; immediately afterward: watched the late movie and then went to bed.

In addition; moreover; besides: It costs $20, and then there's the sales tax to pay.

Used after but to qualify or balance a preceding statement: The star was nervous, but then who isn't on the first night of a new play.

In that case; accordingly: If traffic is heavy, then allow extra time.

As a consequence; therefore: The case, then, is closed.

n.   That time or moment: The bus leaves at four; until then let's walk.

adjective.   Being so at that time: the then chairman of the board. 

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THAN       (thān, thən)

conjunction. 

Used after a comparative adjective or adverb to introduce the second element or clause of an unequal comparison: She is a better athlete than I.

Used to introduce the second element after certain words indicating difference: He draws quite differently than she does.

When. Used especially after hardly and scarcely: I had scarcely walked in the door than the commotion started.

preposition.   Usage Problem

In comparison or contrast with: could run faster than him; outclassed everyone other than her.