"Then" is an adverb that is usually used to describe actions in time, and "than" is a conjunction word used for comparisons. "Then" can also be a noun or adjective that represents a specific time, says Grammarist.
For example, "then" is used to describe time in this sentence: “I woke up, and then I had a cup of coffee." "Than" is used in the following sentence as a comparison word: “My coffee tastes better than yours.”
The comparison word "than" has no synonyms, where "then" can be replaced with synonyms unless it is used in an "if...then" sentence, states Grammarist.