Than compares two things. Then marks time or sequence. They are not interchangeable, and ACT English will put both in the choices. Diction items like this are in word choice on ACT English.
Comparison takes than
The retake felt harder than the first test. She would rather drill math than reread the passage. If you see a comparison — more, less, rather, other — than is the word. Comparison wording more broadly is in comparison words on the ACT.
Time takes then
Finish the section, then check the ones you flagged. Then means next, or at that time. It also shows up in if-then: If the clause is essential, then leave the commas off.
A similar pair, where the words are related but not synonyms, is affect versus effect. When the issue is tone instead of a swapped word, see formality.
The bottom line
Comparing uses than. What happens next uses then.
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