Saying ‘yes’ in French is not always as straightforward as you might think.
The usual word for ‘affirmation and assent’ (as my old grammar book has it) is, of course ‘oui’. You can spice it up a bit by saying ‘ah oui’ or even ‘ah baa oui’ which strengthens it a bit – certainly! Indeed! Of course! You can always repeat – ‘oui oui’. Other words which you can use are ‘certainement’, bien sûr (= of course), naturellement, tout à fait (quite’).
A very useful phrase is ‘je crois que oui’, ‘I think so’ and, by extension, ‘je crois que non ‘ I think not/I don’t think so.’ There is also ‘je dis que oui/non’ ‘I say yes/no’.
However, there comes a time in everybody’s life when you have to disagree – contradiction, correction dissent (as the grammar book informs me). It’s usually in response to a negative question or sometimes a statement. The word for ‘yes’ in a case like this is ‘si’ (which, as you know, also means ‘if’ and ‘so’).
e.g. Vous n’êtes pas anglais? Si!
Ils ne veulent pas venir? Si!
This version of ‘yes’ is a bit stronger than ‘oui’ as it has to overcome a negative and is best translated, in the first example as ‘(Oh) yes I am!’ and in the second as ‘(Oh) yes they do!’
The problem is using the right one when you’re out in the field. Get the wrong one – it doesn’t really matter much unless you’re working as a translator for the United Nations.