The wording of your question implies that you are aware of the concept of "fair use". It is legal to use brief quotes from someone else's work without permission.
A very common question on fair use is, "how much can I copy", a question which the law and the courts answer, "It all depends, and each instance must be decided on a case by case basis". From a legal, peace of mind point of view, it would be nice if they said, "You can freely copy 500 words, but 501 words is too many", or whatever number. But they don't.
Copyright law explicitly says that fair use is broader when material is copied for educational purposes than for commercial purposes, and I think a site like this would qualify.
The point of copyright law is to protect the FINANCIAL interests of the original author, not their reputation or legacy as the smart person who wrote this thing. That is, the key question is, Would someone buy your book that copies large portions of that other person's book, rather than buying the other person's book? If someone is posting their work on the Internet where anyone can read it for free, they are not losing anything by your copying, so fair use should logically be much broader.
So in this case, I doubt it's much of an issue, especially if you only copy a minimum relevant amount. If someone asks, say, about the proper use of a semi-colon and you copy a couple of sentences that describes proper use of a semi-colon, I think you're good. If you copy the entire chapter on punctuation, there could be a problem.