Which/what

[Muchtoomuchtodo]

Please help!

Ds has been saying, "what one?" recently which I've been correcting to which one.

Now he's started replacing 'what' with 'which' every time and it doesn't sound right some of the time but I don't know the rule explaining how to chose the correct word.

Can anybody explain it clearly to me please?

Generally 'which' is for a more restricted possible set of answers- ' which do you want- the blue or the red one?' 'What' when there are more choices..'what's your favourite colour?'
Often they are interchangeable though.

"Which" would go with "which particular...?"

Thanks Sangria.

That helps a bit I think!