When we say a negative sentence, we put the word not after the first verb in a sentence. We do it for to be, can, could, present continuous, past continuous and future simple. When we speak, we often use short forms.
In the present simple and past simple, we use don’t, doesn’t or didn’t, when we say a negative sentence.
The water wasn’t only a drink.
I sat down too because I couldn’t stand.
I didn’t understand why I felt so sad.
You can see more examples in the book The Little Prince.