Hey, I started learning Russian for my degree in September and I am obviously finding loads of things I don't understand.

At the moment I am trying to understand when and how to use the 'ли' partical.

And also, in what situations should you use 'ты не мог бы' and 'вы не могли бы'? I know the first is informal and the second is formal, but in which formal and informal situations should they be used?

Please can someone help me out with these.

Thanks