I saw this:
Is this a correct sentence? I got confused with おしえる人. I thought we cannot follow a verb with another noun without nominalizing it with either の or こと. Should this be おしえるの人?
This is a very important construction called a relative clause.
a person who teaches
The rule is described in detail in this question: Relative clauses distinguishing whom/with which/that
In general, it is safe to use a verb directly before a noun. There is no modification needed. If you add の after a verb, you turn it into a noun. Two nouns cannot be connected directly, there has to be a の inbetween, as in 私の本.