Using 'si' instead of 'oui' to disagree with a negative question or statement

In most cases, we use oui (yes) and non (no) in French the same way as in English.

However, if someone asks you a negative question or makes a negative statement and you want to disagree with it (i.e. answer in the positive) then use si instead of oui.
 
Here are some examples:

Summary:

- Use Oui  to answer "yes" to positive questions/statements

- Use Si  to answer "yes" to negative questions/statements 

Note that non is always the same whether you are agreeing or disagreeing: