If it generates “I ate” and the next word can be “a” or “an”, then it will just generate one or the other based on how often they appear after “I ate”. It hasn’t decided by this point what it has eaten. After it has generated the next token, for example “I ate an”, then its next token is now limited to food items that fit the grammatical structure of this sentence so far. Now it can decide: did I eat an apple? An orange? An awesome steak? etc