Teachers can still come across as being intelligent if they don't know something. I recall asking my English teacher the rules on usage of 'which' and 'that', because I wanted to prove my boyfriend wrong (stupid man dots his paper with 'whiches' that should be 'thats'). She ummed and arred for a bit, didn't give me a definitive answer, and advised me to look it up on the internet. I still respect her immensely, because she's a great teacher, incredibly intelligent, and cares about her students.
Wow--to me, that suggests a shocking lack of knowledge!
No it doesn't. From the description, the minimum that can be deduced is that she was unable to tell what the definition/difference is. She may, in fact, be able to use it correctly but unable to describe the difference. She is not alone. Most English speakers can speak correctly and use "that" and "which" correctly even though they can't tell you what the "rules" are.That which is not proper shall be disdained. That is an object reference - the object shall be disdained. Which is a comparison of objects - proper, not proper. Now that is tricky! Which is why traditional grammar instruction is impotent. We don't learn language through the instruction of grammar contrary to how high school and college texts teach it.