Wow......it's...very difficult.. I'm surprised to..... .. even though I'm native Korean, those two guys are much better than me about Korean grammer!! But. I'm sure about that. 안녕하새요 -> 안녕하세요. And. 못없어. 못없었어요. 못없었습니다. are great example but not existing expression. I think she might explain 멋없어. 멋없었어요. 멋없었습니다. And. The different between 멋없어, 멋없었어요 and 멋없었습니다 maybe..politeness!

The following link also might be helpful, though the table includes few minor mistakes as the imperative present informal high should be 해라(NOT 하세요,) and the imperative present formal low should be 하세요(NOT 해라.) http://www.verbix.com/languages/korean.php

이다 Is the infinitive form of "to be/ to exist..." present formal: -이에요 / -입니다 Present informal: -이야 Past formal: -였어요 / -였습니다 Past informal: -였어 아니다 Is the negative form of '이다', meaning 'to not be/ to not exist'. Present formal: 아니예요 / 아닙니다 Present informal: 아니야 Past formal: 아녔어요 / 아녔습니다 Past informal: 아녔어 멋있다 affirmative form of "to be cool" Present formal: 멋있어요 / 멋있습니다 Present informal: 멋있어 Past formal: 멋있었어요 / 멋있었습니다 Past informal: 멋있었어 멋없다 the negative form of 멋있다, meaning 'to not be cool'. Present formal: 멋없어요 / 멋없습니다 Present informal: 못없어 Past formal: 못없었어요 / 못없었습니다 Past informal: 멋없었어 Hope it really helped you! :*