1) Weak interactions of quarks:
What were two experimental evidences that weak interactions are not
universal for quarks (depends on the quark flavour).
They led to idea of CKM matrix. Could your write down its form
and explain components. Do you know their approximate values?

2) Neutral kaon system and decay to fermions.
What you can say about:
  --> KL, KS transition
  --> strageness oscillation
  --> CP violation