This sentence is grabbed from a website:
A teacher's primary goal is to teach students the best they can about the things that are in our textbooks and more important, how to show respect for one another.`
I am not sure that they in this phrase: the best they can is appointed to whom. I think it is related to "teachers" which came in the singular form at the beginning. (which is wrong and should be plural)
And if it is related to students, the auxiliary can in the following needs a verb?