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:

Agreeing...

Est-ce que tu parles français? Oui, je parle français.Do you speak French? Yes, I speak French.

Disagreeing...

Tu ne t'amuses pas? - Si, je m'amuse.Aren't you having a good time? - Yes, I am.

Il n'est pas gentil. - Si, il est gentil.He's not nice. - Yes, he is. 

Bref, ce n'est pas important... - Si, c'est important !Anyway, it's not important... - Yes, it is!

Tu n'as pas une clope? - Si.Don't you have a smoke? - Yes I do.

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:

Est-ce que tu parles français? Non, je ne parle pas français.Do you speak French? No, I don't speak French.

Tu ne parles pas français?  - Non, je ne parle pas français.Don't you speak French? - No, I don't speak French.

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Learn more about these related French grammar topics

Examples and resources

Tu ne t'amuses pas? - Si, je m'amuse.Aren't you having a good time? - Yes, I am.
Bref, ce n'est pas important... - Si, c'est important !Anyway, it's not important... - Yes, it is!
Il n'est pas gentil. - Si, il est gentil.He's not nice. - Yes, he is. 
Tu ne parles pas français?  - Si! Je parle français.Don't you speak French? - Yes! I do speak French.
Est-ce que tu parles français? Non, je ne parle pas français.Do you speak French? No, I don't speak French.
Tu n'as pas une clope? - Si.Don't you have a smoke? - Yes I do.
Tu ne parles pas français?  - Non, je ne parle pas français.Don't you speak French? - No, I don't speak French.

Attention


Est-ce que tu parles français? Oui, je parle français.Do you speak French? Yes, I speak French.
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