"and" or "or"?

You can use either. You can also use and/or (not or/and) to cover both ideas.

Either conjunction is fine.

What about this?

You can't find words and/or phrases.
(I mean, when you use "not" with and/or )

Using "and/or" should solve your problem.

yeah, I would definitely use that, but I can't teach and/or concept to my students. They have never even seen it used because they are still in high school. So in that case, I have to make a decision which to use.....Which is more preferable to teach?

If you want to teach "and/or", here is a good example:

When you eat in a Japanese restaurant in California, you can ask for American silverware and/or Japanese chopsticks.

teacherJapanWhich is more preferable to teach?

In this instance they are equally good, and a native speaker might use either. You can pick either one to teach your students, or teach them both.

thank you