Simple present

In order to change an affirmative sentence in the simple present tense into a negative sentence, we use do not or does not before the principal verb. Note that do is used with I, you and plural subjects. Does is used with singular subjects.

She works. (Affirmative)
She does not work.  (Negative)
He writes a novel. (Affirmative)
He does not write a novel. (Negative)
I like reading. (Affirmative)
I do not like reading. (Negative)
We pluck flowers. (Affirmative)
We do not pluck flowers. (Negative).
He wants to come with us. (Affirmative)
He does not want to come with us. (Negative)
I always speak the truth. (Affirmative)
I do not always speak the truth. (Negative)

If the affirmative sentence contains the auxiliaries is/am/are, the negative sentence is formed by putting not after is/am/are.

She is a doctor. (Affirmative)
She is not a doctor. (Negative)
I am a teacher. (Affirmative)
I am not a teacher. (Negative)
They are my friends. (Affirmative)
They are not my friends. (Negative)
We are students. (Affirmative)
We are not students. (Negative)
A computer is a useful machine. (Affirmative)
A computer is not a useful machine. (Negative)

If the affirmative sentence contains the primary auxiliaries has/have, the negative sentence is formed by putting do not or does not before has/have.

She has a car. (Affirmative)
She does not have a car. (Negative)
He has attractive looks. (Affirmative)
He does not have attractive looks. (Negative)
I have an Alsatian dog. (Affirmative)
I do not have an Alsatian dog. (Negative)
They have two children. (Affirmative)
They do not have two children. (Negative)

If the affirmative sentence contains a modal auxiliary verb (e.g. may, can, shall, will, should etc), the negative sentence is formed by putting not after the modal auxiliary verb.

He may come. (Affirmative)
He may not come. (Negative)
She can go. (Affirmative)
She cannot go. (Negative)
She should wait. (Affirmative)
She should not wait. (Negative)
You must go now. (Affirmative)
You must not go now. (Negative)
We will help you. (Affirmative)
We will not help you. (Negative)

Note that cannot is written as a single word.