Simple Negation
Making a sentence negative in Spanish is simple. To say that someone is "not" something, or that a fact is "not" the case, just place the word no directly before the verb.
Answering "Yes" and "No"
To answer a question affirmatively, use sí (yes). Notice that it has an accent mark over the í.
When answering a question negatively, you will often hear no used twice. The first no answers the question, and the second no goes right in front of the verb to make the sentence negative.
Contrasting and Correcting
You can use the conjunction pero (but) to contrast a negative statement with a positive fact.
To correct a simple fact, you don't always need pero. You can just place the negative and positive statements side by side.
Evaluating Facts
Here are a few useful adjectives for evaluating facts: cierto (true), falso (false), fácil (easy), and difícil (difficult).
Remember that adjectives ending in -o change to -a when describing a feminine noun, but those ending in consonants (like fácil) stay the same for both masculine and feminine nouns.