미쳤어 is composed of 미치(다) + 었 + 어 었 is a suffix used to express past tense 어 is a ending particle 미치겠어 is composed of 미치(다) + 겠 + 어 미치다 means "mad" 겠 a suffix used to express one’s opinion/idea/assumption = I think, I guess, I assume 어 is a ending particle 내가 미쳤어 means "I was mad" 내가 미치겠어 means "I think I will be mad"