The negative construction is formed differently depending on whether the verb is an auxiliary verb (be, have, will, can, etc.) or an ordinary verb.
• If the verb is an auxiliary verb, the negation is constructed with not (or with the contraction -n't) and without the auxiliary verb 'do/does/did':
She cannot know the truth. It's impossible that she knows the truth. (Note that the words 'can' and 'not' form only one word in the negative; 'cannot'!)
I am not ready yet. I am still not ready. It is too early for me to be ready.
They won't come. They will not come.
• If the verb is an ordinary verb, the auxiliary verb do/does/did is used to introduce negation:
He does not (or doesn't) play rugby. Rugby is not a game which he plays.
They didn't go to the theater yesterday.
The contractions -n't (isn't, aren't, doesn't, don't, won't, can't...) are frequently used in spoken English. Using not separated from the word often allows us to emphasize the negative idea in the sentence:
Is Bruno home? No, he is NOT (at home). Is Bruno home? No, he is definitely not.
The ordinary verb to do also conjugates with the auxiliary verb do/does/did:
He doesn't do his work properly. He does not complete his work correctly.
You didn't do my laundry. You failed to wash my clothes.
Negation can also be introduced by the adverb never, which in itself carries a negative meaning:
I have never seen the Eiffel Tower. I have not ever seen the Eiffel Tower.