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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.
"ELICIT" and "ILLICIT"
"Elicit" means to draw out or provoke a response. "Illicit" means illegal or forbidden. Ex.: The teacher's question elicited a lively discussion. The police uncovered an illicit gambling operation.