1) 화나다 being present tense and 화내다 being past. -> not past but present.
past tense 화났다 / 화냈다
2) but does this mean you would never say 화냈습니다? -> No. You can use 화냈습니다 for past tense
ex) - 나는 그에게 화를 냈다 I was offended with him. (화냈다=화를 냈다)
- 어제 아버지가 화냈을 때 너무 무서웠다.
Yesterday when my dad got angry, I got really scared.
* We generally use 화를 내다 than 화내다 (and same with 화가 나다 than 화나다)
3) I've heard 화났습니다 is frequently used in the present tense, is this correct?
-> Yes it is. The way to use tense is different from languages.
ex) 너 화났어? Are you angry?
존은 나 때문에 화가 났어(We don't know if Jhon is angry still now) And you can say also "존은 나 때문에 화가 나있어(we can know he is angry) Jhon is angry because of me.
존은 화가 나 있고 혼자 있고 싶어해요 Jhon is angry and wants to be alone.
화내지마! Don't be angry! 그는 언제나 화를 잘 내 He is angry all the time