What part of speech is "how" in "He did not know how he ought to behave"?

Question

This is an example under the definition of how in the Oxford Advanced Learner's Dictionary.

He did not know how he ought to behave.

The dictionary lists how as an adverb. I understand this is because it modifies the verb behave, but I think it acts as a noun here.

Suppose we change how to the way.

He did not know the way he ought to behave.

The meaning is still the same, but it looks like the way acts as a noun phrase here.

So, are both how and the way adverbs in this situation or are they nouns? If they are neither, what are they?

Answer 1

It's a subordinating conjunction there.

Merriam-Webster "how" conj. 1a
the way or manner in which
asked how they could help

The substitution you made does mean the same thing. It's a reduction of "the way that he ought to behave", which is a noun phrase modified by a subordinate clause.

Answer 2

He did not know [how he ought to behave].

"How" is an adverb here, but it doesn't modify the verb "know".

The bracketed element is an interrogative clause (embedded question) functioning as complement of "know". The meaning is deontic, i.e. it doesn't mean whether he ought to behave but indirectly questions the manner in which he ought to behave.

Thus, the meaning is:

"He did not know the answer to the question 'How ought he to behave?'"

In your suggested alternant

He did not know [the way he ought to behave].

Again the bracketed element is an interrogative clause. It's not a question of not being familiar with the way he ought to behave, but of not knowing how to cognitively find the answer to the question "In what way ought he to behave?"