adverb
1
BAD : We took a bus to the city centre then caught a train to London.
GOOD : We took a bus to the city centre and then caught a train to London.
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Then is an adverb (NOT a conjunction). It cannot be used to link clauses.
2
BAD : After my father retires, then I shall help him financially.
GOOD : After my father retires, I shall help him financially.
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Then (= at that time) is used after a time clause only when it introduces a result or consequence. Compare: 'When you've seen the trees, then you'll know why I came to live here.' (= as a result of seeing the trees)