when you want to say "no" or "not", it means that you want to make a negative sentence.
I don't know the case at all.
Shanti shouldn't go at that time.
She isn't my sister.
The bold words are called "Negative Notation" or Negation. The way to make a negative notation depends on what sentence are you building? and What tenses do you take?. These two questions influence of what kinds of negation will be. When you say that you want to make a negative verbal sentence in Simple Present tense, then you have to use auxilliary "do" or "does". After that you place "not" behind it.
Do + not === /do not/ or /don't/
Does + not === /does not/ or /doesn't/
Besides, you can make a sentence which uses only "Not" without an auxilliary being used.
I had better not go.
It is wrong when you say : /I hadn't better go./ or /I had bettern't go./
Do what I say, Not do what he says!
In the sentences above, we find the word /not/ without an auxilliary. This is possible to use when there are two sentences which is separated by a punctuation /comma/.
Formula : Sentence-1 + , + not + sentence-2
Or, you can say like ; /Not this time!/
The expression appears when there is a preface coming. Without using the preface, we will not understand why someone says that to us. The preface is, for example, /Let's go now!/. Take notice below!
Andi : Let's go now!
Budi : Not this time!
Those ways are 'standard'. It means that almost patterns use that way.
To express negation, you can use some of words which has hidden negation.
Those words have negative meaning even though we don't use auxilliary in it.
I am rather dizzy.
I never come late.
I hardly can move.
I have few friends.
I have little hair.
I seldom wash my hair.
When /few/ or /little/ is used without article /a/, the words (adverb of quantity) will have negative meaning. /few/ or /little/ means very small quantity. /I have few friends./ means that /I just have 3 friends in my life/. But when you say /I have a few friends (with /a/), It means that you have so many friends, and maybe about 10 or more. The differences (few and little (with or without /a/)) are :
1.Few or A Few is used to express Countable noun.
2.Little or A Little is used to express Uncountable Noun.
The hidden negative meaning depends on the word behind it.
As you know that the word /dizzy/ is negative. And, The positive is /Healthy/. The use of hidden negative meaning (HiNeM) depends on the word which is used behind the "HiNeM". Besides, we can use it when the expression comprehensively contains a negative statement.
I hardly can move. (It is very very difficult to move).
I seldom learn English at home. (Compare to : I often learn English At home)