It used to be that people paid me to write. Paying attention to grammar, as well as other aspects of producing publications, was important; remnants are still lodged in my brain!
Recently I've been noticing the words "then" and "than" being misused frequently by people who ought to know better.
THAN is a conjunction that connects two things being compared. Turquoise is bluer than red. Barack is taller than John. Cinder has more fur than Jumper. This might be more than you want to think about.
THEN is an adverb of time or order. That was then, this is now. What did he do then? Jim burped first, then Nancy chimed in. When you get these two words straight, then what?
If you're interested in reading more about using these two words, this Google search is a good place to start.
Other often confused sets of words are its|it's|its', insure|ensure, and affect|effect. Washington State University hosts a set of pages clarifying the use of hundreds of words. Book and calendar versions are also available.