a no ser que
 

a no ser que : I think it means even if, but it might mean unless. Can someone clarify this? Thanks.

A no ser que means unless.

Haz esto por mí, a no ser que ya te tengas que ir.
Do this for me, unless you already have to go.

That is. I don't think it's complex to use it, as you can always say a no ser que in the same contexts that in English you would say unless.

Eso es. = That's it. :)

Thanks a lot, David. I was doubtful about it, but couldn't remember this small phrase!

Yo que sé! is an expression I hear always, I suppose it means; what do i know? but I´m not sure.

You are right!
It's also very common: ¡Y yo qué sé!

It implies a certain level of familiarity, so I wouldn't use it in a formal context, Jane.
In my mind it belongs to the sort of expressions my teenage daughter uses all day long:
- whatever!
- as if I cared
- Duh!
When you say ¡Yo que sé! you are implying you want to be left alone.

Hmmm...
:coffeebreak:
Point taken.:cool:

What about what do I know? Does it imply the same as yo qué sé?