Replying to n'est-ce pas

Question

Bonjour, Does n'est-ce pas make a sentence negative and thus require si when replying to it or do you use oui. For example, Tu parles anglais, n'est-ce pas ?Is it si or oui, je parle anglais?Merci

Maarten Kamp · Accepted Answer

"N'est-ce pas" does not make a statement negative, it is a form of 'converting' a statement, affirmative or negative, into a question. 
If the statement is in a positive form - eg 'tu parles anglais, n'est-ce pas ?' the choice to answer is between 'oui ...' or 'non ...'.  
 If the statement is in the negative, eg 'tu ne parles pas anglais, n'est-ce pas ?' the choice is between 'si...' (the correct option if you want to disagree with the assertion and note that you actually do speak English) or "non ..." 
https://french.kwiziq.com/revision/grammar/use-si-instead-of-oui-to-disagree-with-a-negative-question-or-statement

Chris Wirt · Answer

N'est-ce pas is just like the English "isn't it" or "don't you" or similar. It requests a yes/no reply to the question. In the positive, the response is oui/non and in the negative it is si/non. But that would be the case even without the n'est-ce pas appended.
Vous venez demain, n'est-ce pas? -- Oui, je viens. / Non, je ne viens pas.Vous ne venez pas demain, n'est-ce pas? -- Si, je viens. / Non, je ne viens pas.La maison est très spacieuse, n'est-ce pas? -- Oui, elle l'est. / Non, elle ne l'est pas.

freakingtv BS · Answer

I love where you went with this. Drives me nuts in English as well when the speaker says something like "Isn't it true that you're 6' tall?" And technically the correct answer for a 5' tall person would be "yes" since they really asked if "it is NOT true", as technically n'est-ce pas means "it is not that".  But in both cases the speaker means "is that correct?"   Colloquialisms!