First one is correct. Second one is wrong. You would say "I should have eaten more."

Please remember that after so-called modal verbs, you ALWAYS use the base form of the verb (the infinitive with NO "to"): 1. I want to lose some weight. I should EAT less ice cream. (But it is so delicious!) 2. It is raining. You should TAKE an umbrella. 3. She has a test next week. She should STUDY hard for the next seven days.

First one is correct obviously.

'I should eat more' veut dire 'Je devrais manger plus'. C'est une phrase tout à fait juste. Par contre, 'I should ate more' signifie 'Je devrais mangé plus' - c'est évident que cette phrase est incorrecte. Si tu veux parler d'un repas auquel tu n'as pas assez mangé, tu pourrais dire 'I should have eaten more'. Cela signifie 'J'aurais dû manger plus'.

"I should eat more" is correct. Ate is past tense. "I ate more today than I ate yesterday."