The Sentence Diagram Generator visually represents the grammatical structure of the sentence "I had just gone into the kitchen to fix some popcorn." Here is a breakdown of the diagram:

1. Subject: I
2. Verb Phrase: had gone
   • Adverb: just
3. Prepositional Phrase: into the kitchen
   • Preposition: into
   • Object of the Preposition: kitchen
   • Determiner: the
4. Infinitive Phrase: to fix some popcorn
   • Infinitive Verb: to fix
   • Object: popcorn
   • Determiner: some

This structure highlights the relationships between the components of the sentence, illustrating the subject, verb, and various phrases.