Simple sentences are generally affirmative--positive--which means they describe a state or action that is true, has been completed, is currently in progress, or will be completed sometime in the future. A negative sentence describes the opposite: an action or state that is false, or did not, does not, or will not take place. Turning an affirmative sentence into a negative statement involves the addition of the word not to the sentence, as well as additional verbs, if necessary. Whether to add another verb, as well as which type of verb to add depends on the form of the original affirmative sentence.
If a sentence contains one or more auxiliary verbs, it may be negated by simply adding the word not directly after the first auxiliary. For example, the sentence I may come over later, may be negated by adding not immediately after the auxiliary may--I may not come over later. Other examples of auxiliary verbs include to be, as well as its other forms (am, is, are, etc.), to do, to have, can, could, will, would, should, should, must, shall, and might.
A sentence that is in the simple present tense, such as I walk to work, requires the addition of a form of to do, in addition to not--I do not walk to work. If the subject is he, she, or it, the word does is used, instead--He does not walk to work. The same is true for a sentence that includes a modal auxiliary (may, can, shall, should, will, could).
If the sentence contains a form of to be, the sentence can be made negative by adding the word not immediately after the auxiliary. For example, the affirmative sentence She is sleeping, not must be added after is to render the sentence negative: She is not sleeping.
Similar to a simple present tense sentence, a statement that contains the auxiliaries has or have must include the auxiliary do, does, or did, in addition to the word not. For instance, the sentence They have the ball becomes They do not have the ball, when it is negated.
Contractions are created when two words are placed together in a shortened form. Contractions are usually used in spoken English. Negative contractions are created by placing a verb with the word not, in a shortened form, and adding an apostrophe. Have not becomes haven't, for example. Am, however cannot be turned into a negative contraction; it must instead be simply paired with not.