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"THEN" and "THAN"
"Then" relates to time. "Than" is used for comparison. Ex.: First, we'll have lunch, and then we'll go for a walk. He is more experienced than I am.
"IMPLY" and "INFER"
"Imply" means to suggest indirectly. "Infer" means to deduce or conclude from evidence. Ex.: Her silence seemed to imply disapproval. From her reaction, we can infer that she is unhappy.