These are all forms of the past simple. 1. I ate .... = affirmative 2. I did eat .... 3. Did you eat? = interrogative 4. You didn't eat. = negative As you can see, we use the auxiliary verb 'did' plus the infinitive/base form (eat) for questions and negatives. The form 'I did eat' is not wrong, but it is less usual. We use this form to give emphasis, or to contradict someone. For example: Mum: You didn't eat breakfast. Boy: I did eat breakfast! I had some toast. Please ask if anything else isn't clear. By the way, you made a mistake in your reply to George. clear = 'claro' clean = 'pulito' !

2. is wrong: "I did eat" is not used, use "I ate" instead. All the others are correct