A very intricate question about which I often wonder as well... a) You can use panmal with close friends, as long as they are not older than you. If they are, you should ask them, but not using panmal in this case does not mean that your friendship is not real. It just means that you accept being the "younger brother" of your friend. U can definitely use panmal with kids, but not with younger people that you meet for the first time. b) Formal polite speech is the normal way of speaking/writing in formal situations, it can be used very often, depending on who you meet where... c) The endings ᄇ니다 and 아/어/여요 can definitely be used in the same conversation. Endings in -yo are more handy, that is you can use them not only with verbs, but also with nouns or endings that do not have a corresponding -mnida form, like: jeo-yo, iss.jana-yo, etc. d) One should use -시- / -으시 or -세- each time you re referring to someone older and/or higher in status, including your parents... Hope it helps... but in any case Korean people are not uniform in using those different speech levels. At some point, I think the foreigner has to make his "own style"...

a) Close friends - I personally hate formalities between friends. It's a bit hard to 'wear your heart on your sleeve' using '요'. Some people prefer to use '요' and others to use it as well. I think this has a lot to do with the difference in the upbringing of each individual. You should ask them if you're not sure about it. "말 놔도 돼요?". Or if they feel uncomfortable with you using '요', they will say "말 놓으세요" or "말 놔도 돼요." b) If someone uses formal polite Korean, it indicates that they have no interest in getting along with the listeners at the moment they're speaking. So it's used in situations you don't need to be 'friendly', like when you're giving a speech in front of groups of people - it doesn't count even if some of them are your close friends. Men and women are supposed to use formal Korean when they are in the army/navy/air forces. To sum up, nobody uses formal polite Korean in day-to-day conversation. Even many clerks don't use formal Korean with customers because it doesn't sound kind or caring to customers. c, d) As Hujok says.

No, you don't need to use it with me, a few years are nothing. People use the honorific infix when they first meet each other. Then they drop it while having some talk together, and finally they start to use 반말 getting more close to each other. The time to take for this whole process varies depending on everyone's personality, situation etc.

a) for close friends, but only the ones at the same age, or younger. but you won't use 반말 for younger people who you are not close or have met the fist time, I believe it's a bit rude. b)yeah, that's true, mem use it more, and normaly for older,acquaintances like at work whoever they are older or younger, or people you met first c)good question, yes you can use, but it's ackward, you should match the form the same,like from니다~ ending to 니다. d)well.. not much, it's very foraml. rather use 세~ 오세요come, 하세요do compared to 오십시오,하십시오