요 or ㅂ니다 is ending of the sentence. for example, 먹다's ending is 다 right? and 먹다 is dictionary form. 먹습니다 is most formal form. so you can speak like that to your teacher or elder people than you and 먹어요 is general fromal form. so you can speak like that to your close people and they are not higher ranker than you. that is very important part. I'll tell you this. 먹습니다 to high ranker than you, 먹어요 to same ranker like you(if you can't sure listener's rank, then we assume they are same rank like us) and 먹어 to lower ranker than you or close people to you. so thoes 3 endings are pretty clear to use. problem is other ending 야, 니 and 지. (1) about 야... you can use it when you difine or introduce something to same ranker, lower ranker than you and close people. ex) 나는 학생이야 = I am a student, 이것은 책이야 = this is a book (2) about 니... it's for question sentence and you have use it to same ranker, lower ranker than you and close people. ex) 저것은 가방이니? = that is a bag?, 너는 이름이 뭐니? = what is your name? (3) about 지...you can use it when you are check about something to same ranker, lower ranker than you and close people in declarative sentence. ex) 점심은 먹었지 = I ate breakfast, 저 반지는 비싸지 = that ring is expensive. and you can use it when you are sure about something to same ranker, lower ranker than you and close people in question sentence. and in this case, mostly 그렇지 is fallowing after 지. ex) 이 사탕 니가 먹었지? 그렇지? = you ate this candy, right?, 너 숙제 아직 안했지? 그렇지? = you didn't do homework yet, right? well, I think 지's usage is might be most difficutlt but practice makes perfect so.... don't worry about thoes ending's usage and practice enough, then you will be good at thoes ^__^

나는 그걸 샀어요. 나는 그걸 샀습니다. These days, their feelings are quite similar. But in very formal situation, you'd better use 'ㅂ니다'. 너 그거 했니? 너 그거 했지? When ending questions in a casual conversation, you can use '니', '지'. It's very casual. So we usually use them to younger people than me or close people. But I have never seen the sentences ending with '야'. Can you give me some examples with '야'?

There are 3 main formality levels Generally, you just drop the formal general ending and get the informal version. E.g. 공부를 하고 있다 ~ dictionary form 공부를 하고 있습니다. Most formal. 공부를 하고 있어요. General formal. 공부를 하고 있어. Informal. The 야 you keep hearing comes from the verb 이다 (to be). E.g. 학생이다~ dictionary form 학생입니다. Most formal. 학생이에요. General formal. 학생이야. Informal. The -지, -네, -거든, -군/구나,... come from special endings that are added to verbs to add flavor to speech. -지요(죠), 네요, 거든요, 군요 also exist so once you learn them you'll know how to form the informal version (generally just drop -요). -군 is more manly, 구나 more feminine. Overall, informal speech is formed by dropping 요, except you add 야 to 이다 and some other minor exceptions. That being said there is a lot more to know about proper informal speech, you need to use the right vocabulary etc. as well. Hope this helps, if you have more questions just ask away

There are 3 main formality levels Generally, you just drop the formal general ending and get the informal version. E.g. 공부를 하고 있다 ~ dictionary form 공부를 하고 있습니다. Most formal. 공부를 하고 있어요. General formal. 공부를 하고 있어. Informal. The 야 you keep hearing comes from the verb 이다 (to be). E.g. 학생이다~ dictionary form 학생입니다. Most formal. 학생이에요. General formal. 학생이야. Informal. The -지, -네, -거든, -군/구나,... come from special endings that are added to verbs to add flavor to speech. -지요(죠), 네요, 거든요, 군요 also exist so once you learn them you'll know how to form the informal version (generally just drop -요). -군 is more manly, 구나 more feminine. Overall, informal speech is formed by dropping 요, except you add 야 to 이다 and some other minor exceptions. That being said there is a lot more to know about proper informal speech, you need to use the right vocabulary etc. as well. Hope this helps, if you have more questions just ask away.