Then vs. Than: The Difference in One Minute
Than makes a comparison; then is about time or sequence. Here is the quick way to tell them apart, with examples.

One letter apart, and they do completely different jobs. Mixing them up is one of the most common slips in fast writing, and it is one of the quickest to fix once you know the split.
Than = comparison
Use 'than' when you are comparing two things. Bigger than, sooner than, more than, rather than. If two things are being weighed against each other, it is 'than'.
She is taller than her brother. I would rather walk than wait. This took longer than expected.
Then = time or sequence
Use 'then' for time, order, or 'if this, then that'. First one thing, then the next. It answers when.
We ate, then we left. If it rains, then we'll stay in. Back then, things were different.
The trick
'Then' and 'time' both have an E; 'than' and 'compare' both have an A. Or simplest: if you are comparing, reach for 'than'; if you are talking about when something happens, reach for 'then'.
The Grammar Checker catches then/than slips a spell-checker misses, and explains which one the sentence needs.
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