The more I teach, the more I run into specific questions for which I don't quite know the answer. I also teach alongside some pretty veteran teachers who comment now and again about students asking difficult questions, which further cements the fact that I'll need to be a lifelong learner, too (I think "lifelong researcher" sounds more impressive).
The most important thing is not to know everything, but in most cases, to know where to look when you don't.
This applies to students, especially. I like to tell them, "I am like a set of training wheels [illustrated on the screen by the ever-handy Google Image Search]. Eventually you're going to go out into the big wide world and you'll be writing papers and doing presentations. You might want to use a word or structure, but you won't know how to use it, and I won't be there to help and guide you."
This is part of why I love using corpora in language teaching. It helps students be autonomous learners, and it gives me answers, and if you're like me, you miss that satisfying search for information you so often engaged in during college and graduate school (maybe you just found this laborious and annoying, so to each his own). The BYU Corpora have a search syntax page, and if you familiarize yourself with it enough, you can find some pretty specific information. For example, you might want to see all derivations of a word (or in corpus linguistics, the lemma). You do this by enclosing a word in brackets to look for all forms of the word. Searching for:
will generate the following list:
[document].[nn*] will look for all noun forms:
Have a good one, and good luck out there.