This Troublesome Pair is one I come across constantly. Interestingly, it features just as much in non-native English writing as it does in native English writing – this is usually very much not the case, with the errors found in each kind of writing not overlapping much at all. I can only think that it’s a typing error rather than a misunderstanding or mis-learning.

Then means “at that time”, “after that, next”, and, as an extension of that meaning, “therefore”. “He went to the park, then he went to the shops”. “You have made a mess of the spelling, then you will find people can’t understand you”.

Than is used to introduce the second element forming part of a comparison – “A cat is usually smaller than a dog”. It is used to express a sense of something happening next in a very specific phrase: “No sooner had the company reinvested in stock, than it went into liquidation” and I wonder, on reflection, if that is how the thans have snuck into the place of thens – maybe there’s an echo of this in the writer’s mind, or it’s a case of a rewrite of a sentence that then goes a bit wrong.

Anyway, it’s not strictly necessary to use the no sooner than phrase, so it’s perhaps best to stick to then for times and than for comparisons.

You can find more troublesome pairs here and the index to them all so far is here.