University Profs should not have to give a quick grammar lesson everytime they assign a paper.
In case you don't know, here's the deal with these words.
Then: used in reference to time. A person did this and then they did this.
Than: This is a comparison. Simple.
My personal favourite is that my Prof thought it would be necessary to let everyone know that you cannot pluralize a word using an apostrophe and then 's'. Last time I checked, it was still adding an 's' or 'es' to the end of a word.
And then there is always the difference between there, their and they're.
University students should know this. In fact, I don't think a child should be allowed to graduate elementary school until they know the difference between these sorts of words. Grammar is tedious, but I learned it in elementary school. Everyone else should have too.