Can "to sit" ever be a noun, say in some other sentence? Yes or no?

Also, if it isn't a noun infinitive, then what is its function?

And let's look at the sentence again:

The teacher told the naughty pupil to sit where she could keep an eye on him.

So, what is: "...to sit where she could keep an eye on him."?

"To speak Spanish fluently is one of my goals."

To speak = is an infinitive phrase acting as a noun (go back to my comment and this time, read the definition of what an infinitive phrase is. The infinitive phrase is the subject of the sentence.

Spanish is the direct object of the verb (the infinitive form, the first entry Jack, in the dictionary for its definition) speak.

fluently is the adverb modifying Spanish, it answers the question, "Speak Spanish to what extent?" Fluently.

Is = linking verb

one = the pronoun one is a predicate nominative (aka subject complement) and it explains or identifies the subject. In this case it is explaining that learning Spanish is just one goal.

We know it is one goal because of the final element in the sentence:

of my goals = is a adjective prepositional phrase modifying "one", and it answers the question, "How many goals?" Just one.

If you think or they have taught you that you can just randomly throw words in and anywhere in a sentence and because you get the main idea and know a little about how the words work together, that settles the matter, then you are mistaken. I can't say it enough: **Every single word in a sentence has a function and a purpose.

I like chicken salad.

I = Function (pronoun) and Purpose (subject)

like = Function (verb) and Purpose [A noun or a pronoun, no matter how many modifiers it may have, cannot make a sentence. The noun or pronoun must act in some way, or something must be said about it. The part of speech that performs this function is the verb] Like a state-of-being verb or action verb would be the PURPOSE.

chicken = Function (adjective [chicken is a noun that can be used as an adjective]) and Purpose (to modify "salad" and it answers the question, "What kind of salad ?" A chicken salad.)

salad = Function (noun) and Purpose (object of the preposition)

So there you have it, Jack. I don't know what you know, but I do know what I know, and what I know is logical and consistent. Between me and you, I am just using what I learned; everything I say can be substantiated by using many grammar sources, books and online.

Let's go Jack!

One = is a pronoun and subject