Student or Learner
I have seen some sentence have "if and then ", and some are only have "if"
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which one is correct above?
Also please tell me where should "Then" be used and not.
Both are fine. 'Then' is perhaps a reminder that the utterance has started with 'if' - it is saying, "in that case...'. The longer the if-clause, the more likely we are to use 'then', In my opinion.