Why is "나는 그녀가 사랑해요" wrong? Thanks :)
사랑하다 needs an object, you love "x", so it needs the object particle.
It looks like you're saying "I love ... she loves ..." but no object to love.
When do you use 저는 and when 나는?
저는 is used formally when talking to a stranger, your boss etc. whereas 나는 is the polite form mostly used in every day conversations with your friends.