Hi

I have quick question considering using shield solely as defensive weapon. If PC uses shield in off-hand only to parry and get armor bonus while using main hand to swing with sword, does attacks should be performed with penalties? Does parry tests should be used with penalties as well?

If they're only using the shield to defend (by providing additional armor to that arm and the body + bonus to parry) then there's no problem. The two-weapon penalties mentioned on page 228 only kick in if you're attacking with both weapons, and even then only apply to the actual attack tests.