This is my first post, I really like this site!

Basically I have problems to distinguish the negation of a verb that is applied to a clause, and a verb that is applied to a negation of a clause. The following is an example that I do understand:

3. I do not want to speak about it
4. I want not to speak about it

In the last case, since the object is an action, we prepend "not" before the "to speak" part. The word "not" stays between the main verb ("want") and the verb in the clause ("to speak"). There is no confusion with (3) because if we wanted to negate the verb, we would negate the auxiliar verb.

But in other cases, when the main verb happens to be negated without an auxiliary (such as "can"), I have trouble in trying to see if there is any distinction:

5. I am not able to tell her -> I cannot tell her -> I [negation of "may"] [clause "to tell her"] -> "I may not tell her"
6. I am able not to tell her -> I can omit telling her -> I [verb may] [negation of the clause "to tell her"] -> "I may not tell her"?

I am not sure if (6) would be written equal to (5), and in that case, is there no way to distinguish it from the former?

7. I [verb must] [negation of the verb phrase "have the pencil"]
8. I [negation of verb must] [verb phrase "have the pencil"]

I think we could also consider new examples (9) and (10) using "should" for "must".

Is there something I am missing, or are there any specific rules for negating a clause?

Thanks in advance