punctuation - Comma in compound/complex sentences

The answer, that you should use a comma in this sentence, comes from the use of "if." In fact, with the use of "if" you don't even need "then."

If you can read this, you might want to answer this question.

The comma here is required, and correct, because of the dependent clause "If you can read this." The word "then" is an adverb that just happens to appear after the comma that was triggered by the use of "If."

Now, if you did not have a dependent clause triggering the need for a comma, you would not insert a comma before "then." For example:

I picked up my paycheck then paid my bills.

This is why many people almost instinctively add "and" before "then" to allow the use of a comma between two independent clauses. For example:

I picked up my paycheck, and then I started to pay my bills.