I come to my realization that I should use the particle が to introduce a new thing that a listener does not know about (and intend to know about). For example:
- Tony: This anime opening is good.
- Lucy: In addition to that, it is my favorite part. (particle が is used).
Tony is the listener, and he doesn't know the fact about Lucy. When Lucy says the fact, particle が is used, not particle は.
Particle が and は are very confusing. I hope someone will help me with this. Thank you.