If you don’t know the difference between “then” and “than”, I’m going to stop reading as soon as you use the wrong one. I don’t care if you wrote the modern incarnation of The Brothers Karamazov, I don’t care whether it’s otherwise interesting or enjoyable to read, and I don’t care if you help little old ladies across the street and volunteer at a soup kitchen three times a month. The instant I see “then” where it ought to read, “than”, I’m finished reading.  If you don’t know the difference, you obviously don’t read, and if you don’t read, then I won’t, either. Specifically, I won’t read what you have written.

Than – This is for a comparison. “My grammar is better than yours.” If you are making a comparison, it’s never, ever then!

Then – Indicates a period of time, or a consequence. “I was young and naïve then, and I believed there was hope for the future of literate humanity.” (period of time) “If you don’t have good command of the language, then I’m not going to read what you write.” (consequence)