What is the difference between 저는 and 나는?
I know 저는 is formal while 나는 is informal.
But I still don't know when to use one of these two.
But I've been told not to use 저는 when talking to someone younger than me.
Do you automatically use 저는 when the verb at the end of a sentence ends with -요 ?
For example :

저는 학교에 있어요

I used 저는 because the verb ends with -요 but I've seen people using 나는 even though the verb ends with -요
Can someone explain why is that?
I'm really confused right now.

Also, when do you use 당신?
Will it be okay to use 당신 with someone older than me?
When talking to someone I don't know and is younger than me, it is also alright to you 당신?

that is correct that 저는 is the formal form of I. but the 요 particle can be used either when you use 저는 or 나는. 요 is not something that only can be attached to sentence that used 저는 in it. even if you use 나는, you can show some politeness towards the partner that youre talking to. for example when you speak with someone your age but you're not close with them, you could use 나는 ...............요. that would be casual but still polite.
so its not wrong if you use 나는 and have 요 in the sentence.


it is ok to use 당신 because it is the literal meaning of "you". it is ok if you use 당신 to the younger or the older people than you. but korean rarely use 당신. it would be more natural if you call them by their job position such as 선생님, 대표님, etc. or you could just call them by their name+씨.
if you dont know their name, you can just call them 아저씨/아줌마/할머니/할아버지.