I am, I will admit, a bit of a "Grammar Nazi." You could even go so far as to call me a pendant, because I always strive to be correct in my use of language. And 99% of the time, I am. Although I prefer the term "Grammar Fascist", since the Nazis were hardly the only fascists around at the time, and why should they get all the publicity?
So I'm going to set some people straight on the difference between 'then' and 'than'. I come across this particular error frequently. And when it's a person who isn't a native English speaker, I am willing to forgive it. If it's just due to laziness, then I feel no particular need to be charitable. Of course, the usual objection is "if the meaning is clear, who cares if the language usage isn't perfectly correct?" Well, I do. Blame it on being an English teacher (or a smartass, the two are virtually one in the same ) Besides, I think being correct in one's use of English indicates an attention to detail and a careful disposition, both of which I find worthy traits. And why shouldn't you strive to improve, native speaker or not?
That said, please don't consider this as snobbery. I just care about the English language and wish it to be used properly.
So when to use 'then' and when to use 'than'? Ironically, 'when' is a determining factor.
'Then' is used when describing some kind of sequence. It could be a simple description of timing, e.g. "I had a bath, then I ate dinner, then I went to bed." In this sentence, the order of events (when each event happened in relation to the others) is being described. "I had a bath and ate dinner and went to bed" provides the same information, but in this case, it's not explicit in which order the events happened. It could well be that "eating dinner" came before "having a bath". Context dictates that "going to bed" should be understood to be the event transpiring last, since it's hard to imagine a situation in which you ate dinner or had a bath after going to bed, but it's still technically possible.
A second use of 'then' is when used as a conditional, e.g. "When English is used correctly, then I am happy." or "If it rains, then I will stay inside and watch ponies". It's more subtle, but the sequence is still there. The clause before the 'then' occurs first, and is the cause of the clause after the 'then', which has to happen second.
'Than' is used when a comparison is being made, e.g. "Rainbow Dash is faster than Lightning Dust." And that's it.
If you find yourself wondering which one to use, remember that comparing needs 'than', when needs 'then.'
If you found the deliberate error in the first paragraph, then you probably don't need me to tell you all this in the first place.
If you found errors anywhere else... QUICK TELL ME SO I CAN FIX THEM!