A practical english grammar - Vyssaja skola 1978

Negation
Sound symbols

English sentences may be either affirmative (they state something) or negative (they deny something). The negative status is indicated by the word not, or more often by the negative particle n't, which has the sound /nt/ after vowels and /г/, and after consonants constitutes another syllable with the sound /ənt/. If the verb is be, n’t is attached directly to the form of be; the rule is different for other verbs. In state­ments, n’t and not occur in the same place; in questions, not must follow the subject.

That man isn’t (s not) my father.

Isn’t that lady your mother?

Is that lady not your mother?

In statements with be, the negative can be said two ways:

He’s not my father.

He isn’t my father.

That lady’s not my mother.

That lady isn’t my mother.

One form of the contraction is as frequent as the other, and speakers shift back and forth readily.