The difference between than and then is simple: Then usually relates to time, and than helps make comparisons.
Then
Then is usually an adverb, but it sometimes functions as a noun or adjective. It’s used to describe (1) when something happened, (2) when something was something, or (3) a time when something happened. Here are examples of the three main senses of then:
[1] Lawmakers would then turn their attention to a financial regulatory overhaul, and then pick up where they left off on health care. [Star Tribune]
[2] The then President Olusegun Obasanjo gave him his full backing. [Punch]
[3] By then, he was increasingly viewed like a precocious child whose manner had soured from cute to insufferable. [Gazette Net]
Then is the correct word in if … then constructions—for example, If the book is good, then you should let me read it. This is the only sense in which then doesn’t necessarily relate to time. It’s synonymous with therefore.
Than
Than is a conjunction that assists in making comparisons—for example:
For Wizards, more questions than answers [Washington Post]
Getting excited about my wedding rather than my marriage was a red flag. [Psychology Today]
