Well always it depends on the context and situation. Usually we don't say starting with 나는 and ending up with 습니다. But in some cases like speech, it sounds natural. For example, President Moon said, '나는 공정한 대한민국을 만들 것 입니다.' In casual conversation, 저는 blah blah blah -요. 저는 밥 먹었어요 If you say 나는 blah blah blah, then the sentence would be ended with -했어. So you want to say between the same age friends or when your opponents is younger than you. 나는 밥 먹었어